I have the same problem and also I have the similar computers Dell optiplex GX620. I don’t have any problem in unicast mode but in multicast I always obtain the “Blue screen” " Please Wait" and in the multicast log this menssage
[06-19-12 4:28:15 pm] * [06-19-12 4:28:15 pm] Checking if I am the group manager.
[06-19-12 4:28:15 pm] * [06-19-12 4:28:15 pm] I don’t appear to be the group manager, I will check back later.

Any of you can help me??
Thank-you very much.