There is no MSN Messenger 8 yet. Not in public beta anyway.
However, there's a new virus going around pretending to be "MSN Messenger 8 Working BETA".
There's two ways to catch it. First, by downloading it from a fake site where it has been supposedly "leaked":
If you download and run BETA8WEBINSTALL.EXE from that site, you won't get a new chat client. Instead, your existing MSN Messenger will start to send download links to everyone in your contact list. It also connects your machine to a botnet server.
The download link always contains the recipients' email address. For example, if you'd have a friend with email address email@example.com, he would get a download link like firstname.lastname@example.org:
We've just added detection for this one as Virkel.F.