If you've already backed up all the valuable information on the PC, then boot up from your windows pc. preferably winxp or 2000, they both have different steps, but if your pc is from HP, you probably have a custom system disk.
Boot up with that then if you hit a d:\ just type in format c: and then follow instructions on screen.
This is however not recommended if your not at all familar with computer lingo. Just take it to someone who knows or contact their customer service due to possible and likly variations in steps.