How do I clear everything off an old computer and restore it to factory specs? So I can give it to someone.?

It depends on what u want to clear, whether is the CMOS settings or Hard disk.

Clearing the CMOS
When the computer is booting, continue pressing "delete" Key (in some computer, you use F8). Use your arrow keys to scroll down to Factory settings)

Clearing the Hard Disk
The best way to clear every thing on hard disk is to format it.
You can use your XP Cd or Vista CD to do it.