I just confirmed that when VPC 6.1 (w/ win2k) is running (under 10.3.2), the built-in OS X firewall claims "other firewall software is already running". As soon as you quit VPC 6.1, the OS X firewall preference pane behaves normally, without the error message given above. I've seen others mention the "other firewall" problem, so I thought this data point might be helpful. I have no idea if other versions of OS X or VPC exhibit this problem. I have no way to test other combinations.