A lot of people hate their PCs. Blue screens, glitches, and performance that would make a crippled octogenarian seem fast by comparison might just make you want to switch to a Mac, or if you're savvy enough, use Linux. Fortunately, just as with cars and people, a bit of basic maintenance can help avoid this. Here are a few simple things you can do to keep your PC in tip top shape, whether it be for playing games or just browsing the web.
1) Reinstall Windows! This is something you should do from time to time, ideally once a year. Now would be a great time to try out Windows 7 RC.
2) Uninstall programs you don't use.
3) Use a disk defrag program such as Auslogics on a regular basis (at least once a week).
4) Keep your PC updated! Use Windows update and update your graphics card drivers every few months.
5) Run CCleaner.
6) Get rid of startup programs. CCleaner can help you do this.
7) Get rid of unnecessary background services. Hit CTRL+ALT+DEL, go to the "Services" tab and hit services at the bottom. Make sure you don't kill any necessary services though!
9) Do Google searches on how to optimize your OS (there are tons for Vista, quite a few for XP).
10) Dust out your PC. Get a can of compressed air or a vacuum and eliminate all those heat sucking dust bunnies.
11) Ditch anti-virus and anti-spyware programs. As long as you keep your PC updated and don't visit any sketchy websites (read: porn/torrent sites) you should be fine. I've been using Vista without any protection aside from Windows Defender for over a year and a half and have had no problems.
12) For very old PCs, try out various Linux distributions. Ubuntu is the most popular but others may perform better.
13) Realize that tweaks and optimization can only take you so far. If your PC is more than 5 years old, it might just be time to get a new one.
14) Finally, have patience and realize that problems are inevitable from time to time. As long as you backup your data there's no need to stress. If you follow these basic guidelines and reinstall your OS on an annual basis, you'll avoid a lot of headaches and have a very fast, stable PC!