Try De-Fragmenting your HD. Also clear any Cache you have in IE, Next try a couple of little programs 1) Ad-Aware, and 2) SpyBot search and destroy. It is very easy to un-intentionally install SpyWare with many share-ware programs.
I have made quite a few PC's run essentially as fast as they ran new by using these simple techniques.
I've got a year old machine with 512 ram and a 1.7 processor. Running windows 98. I use a ad-aware and a firewall. I have to reboot about every 2 days or the internet I.E. grinds to a halt. Any ideas?