Actually, I have an opposite problem.
When I move over the text in conversations, it's highlighted instead of scrolling the conversation. Plus I have a standard scrollbar at the right side of the conversations (like up arrow, then moving bar, then down arrow) and the only way how to move in conversation is to use this scrollbar, which is pretty annoying.
I don't know what caused this, I'm sure it was caused by something I have installed, just I don't know what exactly :-)
So if anyone has any idea, please let me know.

Thanks.