Thats not your PSU. Slim chance it is. ]
Ill try what you said and see if i can isolate it to the mobo alone. Im pretty sure it is the culprit.
I am thinking of getting this upgrade kit for herhttp://www.compusa.com/applications/SearchTools/item-details.asp?EdpNo=1733953&CatId=332
Her old system was an intel core 2 duo at 1.86ghz and 4 gigs of ram.
Im thinking the triple core, 8 gigs of ram, SSD and a fresh windows 7 install ( and maybe luck unlocking the 4th core ) should make her happy.
Id rather build her a new pc then replacing and old motherboard, its about the same work, but building new is always more fun.