Bios wont load from hard drive?

I recently took in a friends computer that his 8yr son uses. Windows was full of viruses so i had to reload windows xp onto the hard drive ( via install disc ) That was a problem in itself, after changing the boot sequence in the bios it would not load from the cd even though it was in first place. After hours of trying different things i gave up and took the hard drive out of his computer and put it into my old desktop and re installed windows without a problem. Now after putting the drive back into my friends pc it wont load windows. All i can do is enter setup mode ( bios settings and what not ) and one thing i did notice is that the cpu time is off every time i unplug the pc ( not sure if this is related or normal ) .. Any thoughts on what may be causing the problem?