The mainboard has recently been changed, could that be the problem?
If you simply connected the HDD to the new build ...absolutely.
XP is running on the old hardware profile.
Travel Here for instructions on a repair install of XP.
I suggest reading it through first to get familiar with the procedure and then printing out a copy for reference...
Warning !: Any and all data can be lost in this procedure so make sure to backup anything important before proceeding.
However if done properly all your data, programs and OS will be as they were.
Good Luck.