I have also just started to get this problem after the last patch, I'm only running windows firewall, and AH is allowed, nothings changed on my computer since it worked fine apart from tha patch.
EDIT, just rebooted and can get in. Last night after losing connection and trying to re log the alt tab box came up and stayed there and it wouldnt let me in.
Just tried now and the alt tab box flashed up for a second, dissappears and I can get in.???