You can do several things depending on how paranoid you are (.....I typically remove the harddrive and remove the platers and then beat the @#$@#$ out of them with a geologist hammer.......I simply can't take any risk of information leaks)
deleting cookies here and there etc might do the trick if you have never stored something REALLY important in the computer and these days chances are you DID store something really important
Reformatting and reinstallating the operating system will minimize problems in 99.9% of the cases
If you want an extra degree of confidence download the program here
http://dban.sourceforge.net/ (its free)
you either make a bootable floppy or a bootable cd with it
once you have made it you will boot the computer from the floppy/CD and then the program will take care of completely writting random info to the harddrive (you can configure the number of passes)....
Thats the only REAL way to make sure noone can recover the info, that is, make sure that the harddrive is completely rewritten with useless info over and over