So your first step in addressing an easily solved problem is hours (or days) of reinstalling and patching after a full format of the system? Seriously? No attempt at trouble shooting and malware removal at all?
---------- Post added 2011-05-22 at 02:01 PM ----------
Of course his system doesnt slow down, it's hogging all the bandwidth. For the sake of curiosity, is his cabled and yours wireless?
Just open utorrent, go to Options --> Preferences --> General --> uncheck 'Start utorrent on startup', and then shut it down, btw it is quite careless having utorrent on auto startup and seeding some random torrents, depending which country you live, you might easily get a letter from your ISP that they will shut down your internet for good if you continue it.
Btw, I got even better idea, If you know your max download and upload speeds, you could change the settings in the bandwidth tab in utorrent preferences so that he can never steal the whole bandwidth.
For example if you got 10M download / 1M upload broadband, you could change the settings to something like 'Maximum upload rate 75' and Maximum download rate 900'