I'm not a professional techie, just an interested layman
First delete all movies, music, and photos in your computer that you don't want/need.
Then go to Control Panel > Programs > Add/remove programs. There, if you see a program you don't want/need, remove it. (Only if you know what they do!)
Clear temporary files (as explained above)
Check your computer for viruses.
Download a free adware/spyware removal program from the internet and run it.
Run the program "Disk Defragmenter". It will be under Programs > Accessories > System tools if you are running Vista. There, select the disk you want to defragment (C: and any others you may have, like D.
If you are running Vista, stick a USB key into the USB socket (symbol like a fork). In the window that comes up, select "Use as additional memory" (or some such. I can't remember the exact message that comes up).
Hopefully, your computer will be running noticeably faster after all this.