Same problem for me
Hello, I have exactly the same problem, did you resolve it ? On my side, I never use homegroup even between win7 machines, only advanced sharing... After a couple of hours, only XP lost the share with Win7.
The only thing I found, it's the use of the command NET SESSION \\xp_computer_name /Delete on the Windows 7 computer that resolve the problem, or simply by rebooting Windows 7.