who or what hit the computer?

[nihil]

OK,

oh, all log files have only records of events after the incident.

Which implies that they were wiped before?

Did anyone else have physical or remote access? Do your brothers run P2P?

Please run this interactive scan (if you can)

http://www.pandasoftware.com/products/ActiveScan.htm

EDIT: How big was the data on the flash drive? If you can PM me with just one file collected from one of the apparently affected machines, I might be able to help............you will doubtless have to zip it

EDIT#2 This is even more comprehensive, but is in beta:

http://www.infectedornot.com/worldwide/

I still have my doubts about malware. The only think I can think of that does what you are describing is a utility called ITweakU.

http://www.archive.org/details/tucows_194347_ItweakU

Check to see if you have that installed. Also look at your startup file and look in task manager for strange applications and processes.

What files were transferred between the two computers and did your brothers do it?

This free utility might let you see if stuff has really been deleted:

http://officerecovery.com/freeundelete/

In particular, look for any items you don't recognise that might have been used to cause the problem.