Boot Partition Corrupted - Repair??

My HP Media Center PC when purchased ~2 years ago came with Microsoft XP
Media Center. I subsequently upgraded to Vista (another horror story). The
other day when I tried to cold boot, I got an error message that the boot
partition at some file location was corrupted and I needed to run the OEM
installion disk to repair. Since the original OS was factory-installed XP
Medi Center, I do not have OEM XP Media Center disks. Since I upgraded to
Vista, I do not have the full Vista OS disk either (I only have the upgrade
version which I purchased recently). I have tried everything I can think of
using the repair disks I created shortly after purchasing the HP Media Center
PC. UNFORTUNATELY, these repair disks were created using XP Medai Center and
Vista is now the operating system. So they do not help and likely only made
things worse. Now running the repair disks only tell me to reformat the
entire C:\ hard drive. Unfortunately, even if I do that, I do not have OEM
XP Media Center disks to use to re-install XP and I am not anxious to spend
~$400 to get the full Vista operating system disk. Anyone have any
suggestions on what to do to repair the boot sequence? I even was able to
reinstall the boot program for XP Media Center but since Vista is now the OS
that did not work either (and probably only made things worse). Anyone
please suggest something! I need to be able to use this PC. Thanks.