Posted by Sameer
Some users seem to be getting an error 8100030d when they try signing into Windows Live Messenger, in some cases after they have upgraded to the latest version. This error can be caused if the cache files get corrupted and the solution is to simply delete the cache.
Click the following link for complete troubleshooting instructions for this error http://messenger-support.spaces.live.com/blog/cns!8B3F39C76A8B853F!13937.entry. (Special thanks to Jonathan)