This one looks pretty bad.  Don’t respond to an email that seems to be like this:

Koobface spreads by sending notes to friends of someone whose PC has
been infected. The messages, with subject headers like, “You look just
awesome in this new movie,” direct recipients to a website where they
are asked to download what it claims is an update of Adobe Systems
Inc’s Flash player.

If they download the software, users end up with an infected
computer, which then takes users to contaminated sites when they try to
use search engines from Google, Yahoo, MSN and Live.com, according
McAfee.

See the story here:  Destructive Koobface virus turns up on Facebook | U.S. | Reuters