Open Question: How can I flush a hard drive that some one put windows 7 on even though the computer can't handle it?

Girlfriends grandma had WIN7 Installed on her rig even after I told her it wasn't going to work on it. Now no older flavor of windows will flush it off the kernel. Quick fix help? Also in bios it won't let me run immediately from disc it freezes at the splash screen for compaq.