Besides reformatting your PC, here's some things to check out:
- Disable unnecessary startup programs (Start > Run > type msconfig and press enter > Startup tab)
- Update your drivers (big one, could be causing problems)
- Do a full system antivirus and antispyware scan, I recommend AVG Antivirus and Ad-aware for free programs. You may want to post a HijackThis log as well
- Disk Cleanup, I think this is what you were looking for. Pick your hard drive, and select anything that can be removed. (Start > Program Files > Accessories > Disk Cleanup)
- And as your friend said, defragging will probably help. Just leave it on overnight. (My Computer > Rightclick your hard drive > Properties >Tools tab > Defragment Now)
Also, post your full system specs. It'll let us help you more.