I've had quite a few problems with the forums lately (and a few other Live ID sites).

At one point I couldn't open the forums at all, on several of my PCs. IE7 would just show me a white screen whilst continually "redirecting" back and forth between a few URLs.

Another time I could get into the forums, but doing anything that related to me as a user (eg creating a post or viewing my threads) brought me to the "you have successfully signed out" page.

On both occasions, finding the cookie for forums.microsoft.com and deleting it fixed the problem.

Cheers,
Matt