A Puzzle

[DX Wombat]

36hrs ago I had a message from my ISP blocking my emails as they said my computer was infected with the Sircam virus. I understand the necessity of this BUT the computer is definitely not infected. The ISP was adamant at first that a chain of infection had been traced back to my computer but once I had run one of their suggested on-line virus checks which proved negative they were satisfied but puzzled and reinstated my email facility. I have run another, different check this evening and the result is still the same NEGATIVE! So can anyone enlighten me as to why they were apparently able to trace a virus back to my computer when it never had it in the first place. They couldn't explain it but I feel their tracing system / method is at fault. Any ideas? (I have anti-virus software which runs every time I start up the computer)