V3.0.21.329 on XP SP2 with active directory domain. I installed it and rebooted. It froze before login prompt appears. I think this is while it is applying group policy. I left it over 10 minutes and then gave up. I uninstalled it in safe mode but even safe mode was very slow. Everything OK again after uninstall.
There were several messages in the event log from the time when it appeared frozen about being unable to connect to computers on the network and Kerberos authentication failure. Also a message saying the driver CMDGUARD failed to load.
I see others have had a similar problem and a workaround suggested.
http://forums.comodo.com/empty-t16150.0.html. I do not have time at work to investigate this and try work arounds.
Also running Symantec antivirus corporate edition and windows defender.