Many often complain about lack of performance on their old PC and while there's a great deal of things you can do, they are often tricky and certainly requires technical know-how, however. For the mortals, regular maintenance is a good step in the direction of boosting the performance.

So to begin with, scandisk/defragment regularly or use Norton versions which are better in my opinion. How often? - once a month if your usage is low, once a week if you use a lot of programs and/or the internet, every time after you uninstall a large program and again after installing a program).
Run Adaware from www.lavasoftusa.com (thanks Rubi =) ) to keep spyware away from your PC and every few days completely clean out your internet files.

When in Internet Explorer go to Tools, Internet Options, delete files, check the "delete all offline content" box, click OK, click the "Clear History" button. Then go to your Windows folder, usually C:Windows\Temporary Internet Files and remove everything left except any cookies you want to keep. Also delete everything in your C:\Windows\Temp folder after ensuring that hidden files are showing (if you don’t know how, go to Windows help index and search for "hidden files and folders, showing").

Note that new windows versions like XP may have different paths to the folders in question.

Finally, the small programme from Microsoft called RegClean is a good way to correct errors in the registration database, if you don't know what that is, nevermind, just run it once a month. It cleans up in the more vital part of your system.
I found a nice site with some instructions included that may be helpful to you, and of course the programme itself can be downloaded there.