The virus uses the Messenger client to send download links to all of an infected user's contacts, and connects the user's machine to a second "botnet" server, which could then be used to install unauthorised software on the machine. Victims are being infected by clicking and installing the BETA8WEBINSTALL.EXE file that is being distributed via these download links, F-Secure said.
From what I can discern, there is in fact NO SUCH THING as MSN Messenger 8. Microsoft has indeed released a beta version of its next-generation instant messaging client (and only then to a select few) but the software in question is not called MSN Messenger 8. It is in fact dubbed Windows Live Messenger. So, if you see anything called MSN Messenger 8 please do yourself a favour and avoid it, because it is not what it seems.