Someone downloaded xp pro on my win 98 pc

Hello,

I have a situation in which my windows 98 pc, came back to me after loaning to my kid to find he thought he could upgrade it to windows xp pro by downloading what appears to be a homemade disc he must have borrowed from someone. I didnt think that could be done, but he says it operated awhile, and now brings it back to me, because he's afraid he may have ruined my pc, (which I did not give him permission to install other programs..grr), cause now when boot pc, it only goes as far as mouse enabled...then at bottom it says to F1 for setup. apparently the sys config has been messed wth, but not sure, since he lost my operating discs, and only has the windows xp pro disk with its key. Do I need go back to original wndows '98, and how, wihout the disks? the info on my pc is:
HP 4473
support ID: 03-7404-0000-9174-0564
system number: D7404A
serial number available, or any numbers off my pc..

how do I fix this? and do I fix it for win 98 or win xp pro??

HELP