According to Computer Associates

Win32.Nimda.B worm (Also known as W32/Nimda.B@MM)
Win32.Nimda.B is similar in function to the original Win32.Nimda.A worm

Win32.Nimda.B differs from the original in the following manner:

• It is packed (with an EXE packer called PCShrink)
  
• The attachment filename in the EML files and emails has been changed from "readme.exe" to "puta!!.scr"
  
• The filename it opens from infected HTML file has been changed from "readme.eml" to "puta!!.eml"
  
• The filename it copies to the Windows System directory has been changed from "load.exe" to "puta.scr", as well as the corresponding change to the shell= line in SYSTEM.INI.

Win32.Nimda.C worm (Also known as W32/Nimda.C@MM)
Nimda.C is a compressed version of the original Nimda.A worm. It is functionally identical to Nimda.A. At the time of writing, Computer Associates had not received any reports of this worm form its customers, but are posting this description in response to customer interest.

These virus can be viewed in there entirety at the Computer Associates Virus Encyclopedia.

"I send you this article in order to give you advice: SirCam, the annoying e-mail worm that simply won't go away, will turn feral Oct. 16.

According to analysis of SirCam's code, every year on Oct. 16 the worm will delete all the files and folders contained on the hard drives of randomly selected SirCam-infected computers. "

"the
worm's nasty little game of chance next Tuesday, when SirCam will begin deliberately selecting victims for mass file deletion from among all infected computers' whose "Date/Time" system settings use the "Day/Month/Year" format. It will not activate on computers that use the "Month/Day/Year" format."

If you would like to read the full article visit http://www.wired.com/news/
technology/0,1282,47476,00.html