My computer Wont get to the loading screen

Hello,
My name is Phil. Im 18 and i do know a good amount about computers. And no i did not kill this one.
Anyway heres my problem. My aunt gave me her old computer and i wanted to fix it because of so much viruses. And it was working ummm "somewhat" good before. It ran and i was able to log in. BTW her computer is Windows XP. I have a vista CD so i plan on installing it. But heres where the problem comes in. I've wiped a set amount of data off the hard drive cause they were useless and taking up space. I think some were even viruses. She has vista optimizer 2008 on here computer which i know is a viruses or trojan. So i had to turn it off at night cause i wanted to sleep. When i woke up the next morning the damn thing wont even get to the "Windows XP" screen. So you could imagine my frustration. SO What i really want to know is, is there a way to wipe the hard drive COMPLETELY without destroying it with a damn hammer or magnet. Any advice would be great.
Oh and no i will not pay to get it fixed. Cant learn anything by paying for someone else to do it am i right?