This has been happening the last two days. I can't think of anything that changed except I installed Windows Media Player 9.
iTunes for whatever reason will end up in an infinite loop of beachballing. Killing it and restarting it will result in the same, and the window won't come up. Several other programs will also start beachballing and become useless. Logging out and logging back in, the Finder freezes halfway through loading. The only solution is a restart.
What in the hell?