no "one" is sending mail using Bill's Addy. It goes a little something like this, Bill opens an attachment that he shouldn't, wrapped inside of the .exe is a virus of some sort, in this case the Magistr Worm. What this now does is infect 32 bit PE tyle files in the Windows directory and subdirectories. It will then proceed use our POP mail service to email Bill's contact list. And finally to give itself some virus security, it installs itself to run at eash system startup.
The code is encrypted, polymorphic and uses anti-debugging techniques to make it difficult to detect. It has been cleaned before, but for "some" reason he got it again. Dont' fret, no one is infected besides bill,,and if you are infected from Bill's email it is because you opened something you shoudln't have.