Product key w/o installation?

I have recently run into a problem. I have an older computer that got infected with a virus. It wiped everything out. I now am trying to install my xp pro back onto it but have realized that I lost the key. I understand that there are ways to find a product key while the program is installed on your comp but I cannot install it without the key. Is there anyway that I can find out what the product key is? I bought the program legit through school a while back. If I call MS and give them the serial number and stuff on my comp would they be able to see the version and possibly the key for the XP that was installed on my comp?
Thanks for the help.