On certain apps, allowing the firewall alert causes the entire system to freeze, i can move the mouse for about 5 seconds before that freezes too.
It so far has only happend with 2 programs, xfire (after first instaling cis) and company of heroes (The first popup i got for it was fine, but i think the 4th time i ran the game, then quit, i got a second pop up for it which caused it to hang.
Temp workaround is to add the program to trusted apps before the alert has a chance to appear.
Vista x64 sp1
Using default settings