Vista - Windows Live 9.0 won't sign in

I've been having this problem for a few weeks now. After a mandatory update from Microsoft, I've downloaded, installed and tried to use Windows Live Messenger 9.0. But it won't let me sign in. I type my username and the correct password, the icon spins, then - nothing. No error messages, just nothing, the same screen.

I've tried un-installing it, re-installing it at least 5 times. I've tried re-installing and using my old version (Windows Live Messenger 8.1), but it won't let me use that, because it forces me to update to 9.0.

I've already checked a thousand times and it's NOT a problem with a firewall or internet connections or anything of that sort. I've already searched the internet for hours for a solution, but couldn't find anything. I'm using XP and Service Pack 2.

Any help would be much appreciated.