Hi folks. Hoping somebody can help me out here.

I recently bought a used computer for my son on ebay. I was under the
impression that it would come with the HD wiped and I would install my own
OS (Win98 w/ an XP Home upgrade. But when the computer arrived, I found it
had XP Pro installed on it already. At first I thought "Great - will just
stick with that". Problem is the unit was pulled from a school when they
upgraded and therefore, pretty much everything on it is password protected
(and there ain't much chance of getting the passwords from the seller). I
can't even get into the "Start" menu (clicking on the button does nothing).
So I figured I'd go back to plan A and reformat the HD and install my own
O.S. But I can't even seem to do that. I can't even get into the setup menu
on boot-up because that's also password protected. I've tried booting with a
start-up disc but it doestn't seem to be recognizing that.
I'm relatively new to working with XP and although done reformat &
reinstalls before, I've never done it with an XP system. So I don't know if
I'm overlooking something relatively simple or if there's a way to get in
there or what. Anybody have any ideas or help they can offer?
Thanks in advance for your help.