Here is a write-up from Trend Micro:

This new BAGLE variant is very similar to PE_BAGLE.N but bigger in actual size. It propagates via email with varying subjects, message bodies, and attachment file names. This virus searches for files with certain extension names, from which it gathers target recipients. Using its own SMTP (Simple Mail Transfer Protocol) engine, it sends out email messages with spoofed return addresses and itself as attachment.

http://www.trendmicro.com/vinfo/virusencyclo/default5.asp?VName=PE_BAGLE.P

Don't open attachments - disable your preview pane. Download Mailwasher - so you can SEE your e-mails as they are still on your server.

http://www.mailwasher.net/download.php