I'm pretty sure you accidently deleted some kind of important system file when you were cleaning out the hard drive. That happens to me quite a bit, because i always get a bunch of unorganized programming projects, music, and school work all over the place, so when i go and delete it, all of a sudden something crashes. Your best bet would be either to reinstall every driver for the system, or try something like, check every process that his brother is running through the task mananger, and find out which task isn't running on this computer. That might be the link to the problem. If all else fails, backup all the data and reformat the system.
PS. Nice timing, you put your post up at 1 AM.