As we are not the forum in question, I don't think that we can help you directly. However, check your IE settings and reset them to their defaults - see: Internet Explorer - Reset
You may also need to check the other links as well.
If that doesn't help, then I would suggest that you contact the administrator(s) of the forum in question to see if they can come up with anything. It is possible that it is a coding error at their end or that the forum interface is not fully IE7 compliant - the default browser in Vista is IE7, whereas in XP it was IE6.