There are indeed plenty of things you can do!
First off, start by making sure your system is free of spyware, particularly if you use the Internet a lot, as a build-up of this can slow down your PC as well as your Internet browsing experience. As a benefit, you'll be less likely to get pop-ups caused by this malware too. For excellent free software try:
http://www.lavasoftusa.com
http://www.safer-networking.org/en/index.html
Get rid of old files that you don't need anymore, such as recycle bin filed, temporary files and whatnot. Personally I use CCleaner for this (use it once per week or so, and check for updates to it every now and then):
http://www.ccleaner.com/
Change some of the more advanced settings on your PC easily and safely using a special software package. For this I use TuneXP:
http://www.driverheaven.net/dforce/default.php (Info & Download Link)
Lastly, as always, run a ScanDisk (or CheckDisk as it's known in WindowsXP) making sure to check the options to repair any damaged files or sectors. Also, defragment your hard drive if you haven't already (information on how to do both of these can be found by using Windows Help & Support and entering keywords like "Defragment" and "Disk Error Checking".
Just to note that all of the software I've mentioned I use myself and it's all completely free (you don't have to register or sign-up for anything to get it either).