"Antivirus experts are warning of a new virus, code-named W32/Sobig.A, which was discovered late last week and spread rapidly over the weekend. By Monday morning, Sobig was the second most prevalent virus on the Internet, according to e-mail security firm MessageLabs.
Sobig is a mass-mailing worm incorporating its own SMTP engine, according to antivirus companies. It arrives from the e-mail address "big@boss.com" and bears a subject line such as "Re: here is that sample", "Re: Movies", "Re: Document" or "Re: Sample". The e-mail contains an attachment called "Document003.pif", "Sample.pif", "Untitled1.pif" or "Movie_0074.pif".
It affects the Windows 95, 98, Me, NT, 2000 and XP platforms. The worm was originally not considered a serious threat, but has been upgraded due to its rapid spread."
More here.