One of the things I frequently read about is people rummaging around the innards of their computer in a hazardous manner. There are two considerations: not zapping the computer, and staying alive yourself. Computers, like many other electronic devices are not fond of being zapped by static electricity, nor having parts of their circuitry shorted out.
This document is about how not to kill off yourself or the computer; it's not really a “how to modify your computer” set of instructions. That sort of information really depends on what it is that you're trying to do.
This document is only an introduction, and should not be inferred to be the ultimate set of instructions for safety. Do not take unneccesary risks, no matter what you've read, and use your common sense. I will not be responsible if you get into trouble.
Do not “earth” yourself! That's the most dangerous way of setting yourself up to be electrocuted. If you connect with power, you will not have much ability to disconnect yourself. Likewise, don't work somewhere where you might get stuck. Sometimes, the only way to stop yourself being electrocuted, is to back away from whatever's contacting you until it breaks contact (it's a good survival action to learn to rip your hands out of an electrical device at the first sign of strong pain, and step backwards; if you can learn to do that automatically, then you stand a better chance of avoiding electrocution; it's easier to survive a short shock, than a prolonged one). This is hard to do if you're stuck in the corner of the room. And it's probably not a good idea for novices to attempt this sort of work without someone else being around, just in case.
Avoid electric shocks by disconnecting things that you are working on from the mains power, completely. Don't just switch off the power, pull the plug out too! It was not uncommon for TV repairman to unplug the power, be working in a set, and for the owner to plug the power back in, without them noticing; so many techs will take the power plug with them too (this also prevents accidents from unplugging the wrong device). Also, various devices do not switch off fully, or totally isolate the electricity supply, when you use their off switch. And always remember that certain devices (particularly cathode-ray-tube monitors and switch-mode power supplies), can store a lethal charge for a very long time (sometimes weeks).
Avoid static electricity buildup and discharge, either is just as hazardous to sensitive circuitry.
Static electricity can be quite destructive to sensitive computer parts, though often not immediately. Static electric shocks will often damage components in a way that takes a while to show up—long past the time that you'll associate the failure with the exposure to static shocks. So people get quite complacent about properly handling things, as they haven't experience static electricity killing things off (as far as they're concerned).
There are obvious things, like avoiding working on carpets, and moving about as little as possible, etc, but that's often impossible. So I'll discuss the the sort of things you can do easily.
To prevent the buildup of static electricity, you can keep the device grounded. But don't do this in hazardous ways (like leaving it plugged in, or running bits of wire into mains plugs). It's better to leave it connected to something else that is earthed, and switched off, like the video monitor, or some other device that is connected to the computer and grounded.
Anti-static mats can be used too. These are conductive mats (with a very high resistance), to drain away static charges slowly. They're conductive enough to drain away static charges, but not enough to short out operating circuitry. Anti-static wriststraps (that you wear, to keep you discharged) also work the same way (and they're usually connected to the anti-static mat too). They provide a high resistance static charge drain path to earth for you. The mats come in various sizes, from tabletop mats, to sit the equipment you're working with on top of; to floor sized ones, to cover the whole work area.
If you must discharge yourself in a fast manner (because you don't have these anti-static products), then touch something else that is grounded (not the computer) before you handle any parts. And to keep yourself from building up another charge, just rest yourself against something that's grounded (like leaning your arms on the computer case as you're working on it).
When you're handling circuitboards, avoid sticking your fingers onto the components; try to handle the cards by the edges. And when pressing cards into slots, avoid stressing the board too greatly. If the motherboard is going to bend while you push a card in, then place something non-conductive underneath the motherboard to prevent it (also be sure that cards get fully inserted). If you're re-arranging cards, rest the unplugged ones in contact with the computer, so they're less likely to acquire a different static charge build up.
But while handling cards by their edges, keep your fingers off the edge-connector contacts, hold it by the other sides. Your fingers can contaminate the metal, and cause it to become an unreliable conductor, as well as you risk static electricity damage by making direct contact with the electronics.
It doesn't take much to kill an electronic device, so be sure to take precautions before turning a device back on, or operating on it while it's switched on.
One loose screw inside the case could be enough to destroy the computer, if it fell into the wrong place. So be sure to keep track of screws, and if you do drop one in it, be sure to find it, no matter how difficult, before you turn the device back on, even if that does mean disassembling absolutely everything.
Likewise, be very careful not to touch things with the shaft of a screwdriver. Even when switched off, some components will hold a charge for a significant period, or have continuous battery power, and shorting them out with a screwdriver can kill them off.
Properly assemble cabinets. Enclosing electrical devices makes it a lot harder to accidentally get an electric shock.
When disassembling, keep track of what screws go where. Many devices will use different screw types throughout the chassis, and only the right ones will fit properly in the right places. Forcing screws in, and/or overtightening them, can strip screw threads; and lead to screws falling out, and cabinets falling open. To reinsert a screw, carefully place it into the hole, and gently rotate it in the unscrewing direction, until it clunks into place; now it should easily thread back into its original position, like a bolt, rather than like a self-tapping screw biting a new thread.
Be sure to connect cables around the right way. Although most of the connectors have plastic interlocks to prevent them being plugged in backwards, some people hack them off, usually because they've connected one end up wrong, and didn't bother to fix it properly (they connect the other end wrongly, too, to match).
Ribbon cables are supposed to have a trace line (usually red), painted along line one, which should connect to pin one of the plug, if it's been made up correctly. It's also usual practice, for pin one of a ribbon connector to be near the power connector of hard, and floppy, disk drives (though this is by no means standard on all devices, floppies are particularly variable). It's a good idea to also check to see if there is a number one, or some marker symbol, printed near one end of the connector.
It's a good idea to remember that various mechanical devices, don't like being bumped about much (e.g. CD-ROM, hard-drives and floppy-disk drives). So always remember to be careful, particularly with hard-drives, and CD drives, especially if they have media in them (like a disk stuck in a CD-ROM drive), and some of them are more perceptible to damage if they're upside down at the time.
Various drives are only designed to be operated in certain positions. Running a hard drive upside down can be a recipe for disaster, and many must be kept within a few degrees of being horizontal, or vertical. Read the specifications for your drive, check the manufacturer's website if you don't have them on paper.