Gurney -

What you describe sounds like a fairly common happening. If a virus/worm gets on your machine and gets a chance to do anything, often the program bits it installs don't contain a viral signature, so deleting the original virus doesn't do a complete removal. The "installed stuff" looks like any other program, even though malicious code may have been added to a real program. The malicious code can be sent to others, but without the original virus it usually can't send itself to others.

Most AV programs rely on detecting and deleting (or putting in quarantine) an infectious file before it gets a chance to execute anything. If something new gets on before your AV signature files are updated to detect it, the AV may delete the "carrier" when you do an update, but won't necessarily find all the results.

Just one of the reasons why weekly (at least) updates are really necessary with any AV program.

John