Hi bego...the thing with these kind of crashes, as has been said before, is they are extremely hard to nail down. I highly doubt it's the USB causing the issues...I've never seen a logitech keyboard cause the computer to restart...unless it's a problem somewhere else in the computer.
So far, we've tested the memory - however, this isn't a sure fire test...as i've seen faulty memory that hasn't shown up with memtest...and it's pretty much the only program you can test it with. The next step would be Lebowski's first suggestion...power.
If your computer is receiving a bad power spike, it could be enough to restart the computer without even thinking twice. What brand powersupply do you have?
What we can do however, is try and find out what is causing your crashes.
Can you please goto System Properties -> Advanced -> Statup and Recovery Settings -> and uncheck the
Automatically restart box.
Now make your computer crash. If what i think is happening IS happening, then you should now see a
BSOD with a stop error. What is this stop error?