1) Disk Cleanup (start->accessories->system tools->disk cleanup)
2) Defragmenting Your Computer (start->accessories->system tools->disk defragmenting)
3) Removing Spyware & Adware
Here are 2 adware/spyware removal programs that are simple to use (and both are free):
Ad-Aware http://www.lavasoftusa.com/support/download/
Spybot http://www.safer-networking.org/en/download/index.html
4) Uninstalling Old & Unused Programs
(start->control panel->add or remove programs)