First hunch:
Reinstall network drivers.
Switch to Azereus.
Disable sleep mode.
Update your OS.
Not all nics are perfectly compatible with all P2P programs, you might run into probs, also you possibly have not installed the needed drivers for your nick at all or correctly (Though I have no clue how that is even possible, people happen to do that).
It is a driver malfunction you encounter and you need only to find out which driver it is that causes the problem (kinda hard, but definite through the link, if you can not solve your issue, by clean deleting and reinstalling every driver of your system):
http://support.microsoft.com/kb/818501/en-us