It is COMPLETLY ROUTINE for viruses and spammers to put fake From: addresses on their emails, and for those fake addresses to be real addresses harvested usually from the hard disk of the infected machine.

Any ISP which is dumb enough to believe that one of their customers is sending out viruses just because the recipient saw that customer in the "From" address of an infected email doesn't deserve to be in business.

However, an ISP is perfectly capable of scanning outgoing email for viruses and/or spam too, so their assertion may be true. But they definitely should be able to tell you what the virus is - that's very strange if they can't or won't.

Anahata