One of the more common reasons I've looked at someone's computer is because it's been stuffed up by malicious software, and probably the next most common reason is that the owner's stuffed it up themselves. In either case, what's usually called for is more vigilance by the owner with a few things in particular:
Don't use your computer as the administration user. Always log-in as an ordinary, limited, user, without any extra priviledges, unless you absolutely have to be the admininstrator. Switch to the higher-powered admin user to install or reconfigure software, temporarily, then go back to being yourself (log out, then back in again, as different users; or use the switch user feature). In some cases you don't even have to do that, you can just right-click on an install file, and pick “run-as”, and run just that program as another user (the admin user). If you find that you have to make yourself an admin to get some things done, change back to being a limited user when you're finished. Also consider that you should be getting rid of any crap software that requires you to do that, it might be the cause of you having further problems. Running as a limited user makes it much harder for malicious software to sneak onto your computer, running as a super user makes it very easy to get stuffed up.
Don't install everything under the sun just because it sounds cool. You probably don't need half the cool things that you hear about, and probably don't use half of the ones you've already collected. Not to mention that many cool-sounding programs aren't always what they claim to be (e.g. things that install spyware, even fake anti-spyware programs that actually install spyware or other malicious programs), and some are just so badly made that they make your system unstable, even if they never set out to attack you.
Do some research about anything that you're considering installing, to find out what other people have said about it, what problems they've had with it, etc. Type the program's name into a search engine, and tack on other search words like; “review”, “problems”, or “removal”, and search for alternatives.
Treat everything with suspicion. Don't take all comments about programs as being truthful (whether good or bad comments). Always scan what you've just downloaded, or received from someone, for viruses and other nasties. And do this immediately, before you try installing it. I'd also recommend scanning what you've just installed, before you use it, just in case your scanner couldn't check inside an installation package.
Regularly scan your system for nasties. What you thought was safe yesterday might not really be safe, because your scanning software didn't know anything about it at the time.
Keep your protective software up to date (anti-virus, anti-trojan, anti-spyware, firewall, etc.). Things like anti-virus software should be updated daily (i.e. each time you hop on the net, and before you do something that puts you at risk—such as fetching your mail, or using Microsoft's Internet Explorer).
Use anti-virus software and firewalls all the time, they do help you. Once you've set them up properly they don't really cause you problems. If your firewall does drive you mad, you've probably got it configured wrong (not allowing your normal programs, or it's stupidly alerting you about every little thing on the internet that it's disallowing), or you've got something on your computer that you should be concerned about (when some peculiar sounding program wants internet access, you've probably got a virus or trojan—you should get rid of them, not your firewall).
Learn how to use your software properly. Check out what customisation options you have, and set them to suit yourself. Turn off features that nag at you, annoy you, slow your computer down, or breach your privacy. In particular disable options that keep a program running all the time, just run the software when you actually need it, but don't turn off the features that protect you (i.e. don't ignore warnings).
Read the warnings. When you're presented with information before installing software, or allowing it through your firewall, etc., read what it says, think about what it implies, and proceed with due care.
If you ignore the warnings and safety advice you will suffer problems with your computer. There is no avoiding this, you will keep having problems, you will keep on bringing your computer back to get it fixed, again and again; and you will keep on wasting your time and money. Any time I've come across someone who doesn't use protective software, their computer has been riddled with rubbish; and it's generally the only reason that their computer hasn't been working very well for them.
Replace bad software with better software. For example, things like Microsoft's Internet Explorer are riddled with security holes that can affect your entire computer. While no software is perfect, there's much better alternatives, and browsers which aren't so tightly integrated with Windows don't share their flaws so badly (e.g. when other browsers crash, only the browser crashes, not the entire computer; and they're not so easily exploited to secretly install malicious programs on your computer). If you like the all-in-one approach (browser, news and mail with the one program), then Mozilla is a much better alternative. But if you prefer separate programs, then use Mozilla Firefox instead of Internet Explorer for web browsing, Mozilla Thunderbird or Ritlab's “The Bat!” instead of Outlook Express for mail, and Forté Inc's Agent or 40tude Dialog instead of Outlook Express for usenet (news groups). Opera is another decent all-in-one alternative to all of those. Avoiding Microsoft's Internet Explorer and Outlook Express, and anything else that uses them behind the scenes (e.g. like Incredimail, and various other things which pretend to be web browsers, but are merely just a new front cover for Internet Explorer—such as Avant Browser), is the single best thing you can do to help protect yourself from accidents that screw up your computer.
If you only use your machine for work reasons, you might consider ditching Windows, completely. You can use something much more robust, like Linux or BSD, to do ordinary tasks like e-mail, web browsing, writing documents, etc., in a safer environment. It's not too difficult, just a bit different, and easy enough to adjust to (no worse than all the hoops Windows makes you jump through). Of course it's not a guarantee, nothing is completely foolproof, but I've found it survives unskilled user-abuse better than Windows does. It's capable of more than just office work, you can play games and so on, as well, but that's a little bit more difficult to manage.
Amongst other things, there are links to software for different types of computers in my links section (in there you'll find links to quite a few things that I recommend). And the computing section of my website discuss various issues about computers (have a look around, things are organised into topics to make it easier for you).