Yeah, there's an odd bug in IE7 that causes that. I haven't been able to find the cause of the bug, but there is a workaround.
My recommended solution is to choose a new "Thread Layout" other than "Standard".
That more or less takes care of that problem. Also, make sure you head to Admin > Forums > Misc Forum Settings and choose a different default Thread View as well for everyone.
I've been wrestling with that bug for a while now.
Damn you´re fast on answering, thanks alot and will do it that way then.