I have ditched Windows Messenger, and I now use MSN Messenger 6.0 preview.
The main reason was because I feel 6.0 has a better interface with a lot more features.
MSN messenger 6.0 installs over MSN messenger 5.0, but I don't think it touches Windows Messenger (but don't hold me to it!)
I used the add/remove programs tool to remove Windows Messenger, and everything seems fine.
So in my opinion, get MSN messenger.