64 bit windows?? it wasn't even out yet... it is still on beta.
if the disk works fine on the other PCs, then it is the new computer that is having problem.
I won't disregret the problem of virus though... did ur brother put in any disc, CDs u didn't know??
and if it is a new computer, maybe one of its parts is defected... loose cable or something.... I am working in a computer assembly store, and u will be surprise how PCs are assembly, hehe