Thanks for reporting back in bjproc. I'm sorry I didn't get here soon enough to help you because I could have linked you to the fix for this sooner. I'll post the fix for anyone reading this.

If you are having problem joining the Homegroup from certain machines try the following procedure.

Open up a command prompt and type "regedit" without the quotes then hit enter. Navigate to the following key.
[HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Tcpip6\Parameters]

You have to set the DisabledComponents Dword to 0

Very important> If there isn't a disabled components dword you have to create one and then set it to zero. To do this follow the next step while you are still at the same spot in the registry.

In the right pane, right-click and select New – DWORD (32-bit value) and type in it's name as> “DisabledComponents” and set its value to 0.
“0″ = Enable All IPv6 Interfaces (Default)
After that reboot your machine and see if the problem is fixed.

This fix is also outlined in this Technet thread.
HomeGroup get an error of must have IPv6 enabled

It's also a standard fix in the Homegroup walkthough which I have posted many times.
Ultimate Troubleshooting Guide for Windows 7 HomeGroup Connection Issues