Forum Moderators: open
Dan
W3C Validator - HTML [validator.w3.org]
W3C Validator - CSS [jigsaw.w3.org]
But more likely, it's a javascript error of some kind. Go into Tools > Internet Options> Advanced and check your settings to be sure that the script debugging and noification options are not disabled, then see what you can track down.
There's always a chance that it could be another application you are or aren't running at the same time, particular anti-virus and anti-spyware programs. This kind of thing can contribute to the difficulty in repeateing the problem.