You'll need to do a factory restore and this will mean losing all files and programs on the computer. Typically the software needed to do this is stored on a separate partition on the HDD, so hopefully you didn't delete that when you installed XP over 7. If you did, you'll have to try and get restore media from HP.
I have a HP Pro PC that was downgraded from Windows 7 to Windows XP professional a couple of years ago. However I need to undo this downgrade and have the original Windows 7 OS on it. How do I go about doing this? Is there a way I can re-set it?
(I have no disks or anything, just the computer itself).