i had a similar problem, same symptoms but not win 7 related though. it was a comcast issue. my problem was fixed by cloning the mac address of the computer when connected directly to the modem.
Has anyone been successful with this? I can easily mount folders/drives on my mac, but from Windows 7, my Mac isn't showing up on the network. This was ridiculously easy with Vista, but i think this 'Homegroup' situation is messing things up.
iTunes sharing works on both computers, so I know...