If you are running W7 there are a few things you can do - is it 64-bit or 32-bit?
Can you stand it if your desktop looks like an old Server OS or Windows 95 or do you need it to have all the fancy bubbles & Aero features?
Right click on My Computer and select Properties. On the left select "Advanced System Settings". Next click "Settings" under the Performance section (this is on the Advanced tab which should default to being on top).
Select Adjust for best performance and then hit Apply & OK. See if you can stand that. Most people hate it but it is a bunch faster.
If you like that and don't need some fancy screen saver or desktop background you can also disable Themes which saves you some system resources.
To disable Themes you right click on My Computer and select Manage. You'll need to be an Administrator for this.
In the Computer Management console you select "Services and Applications" then "Services" and find Themes. just hit stop & disable, click apply and hit OK.
On restart you'll notice a big difference with both adjust for best performance + disable themes.