Don't know if anyone else has noticed this, but Messenger 2009 seems to start using a lot of memory after a while. I have a 512MB RAM Windows XP computer, and after Messenger's been running for an hour or two, the whole PC keeps freezing, even if I close everything else. But I've only noticed this quite recently, and I haven't installed anything in a while.
I've tried using WLM Safe (which has a memory reduction feature), and that seems to work for about half an hour, before it starts freezing again. Had a look in Process Explorer to check the memory usage, and the "Private Bytes" seems to reach above 500000K.