might be a million miles off beam, but
Was the previous incarnation something like a DELL or Packard Bell machine? If it was, there is a possibility that the Root area of the disc, which it has to access as part of the boot routines, might not have been correctly set up for XP when you did the upgrade.
In some cases, this area is "hidden" from the operating system, so it doesn't show up as a drive as such.
I've seen some very strange things happen as a result of these "hidden" partitions, depending on how the upgrade was installed, especially where there's been a change to a new operating system.