Windows 7 Windows 7 and Vista Share Network Crash

I have just installed a Windows 7 Professional x64 PC on my home network and from time to time the system freezes / crashes. Having finally worked my way through to the event Log I notice that the crashes coincide with a message stating that a family member's laptop running Vista (on the same physical network but obviously not in the Win 7 homegroup) is a making a "master bowser" (sic) call and claims to be in control of the Windows 7 homegroup) - this coincides withe critical message in log. I have checked both systems with up to date AV scans etc without finding a cause.
Any advice gratefully received - I have currently disabled the Windows 7 homegroup settings on new PC - and crashes with that symptom have stopped.