Richard -
If you have scanned your computer with an updated anti-virus program and it didn't find anything, your machine should be clean.
What you are seeing is the backlash of the latest viruses that are spreading. Most recent viruses use a technique known as "spoofing". When a virus infects a computer, it locates all e-mail addresses it can find on the computer. This includes addressbooks and even web pages the person has visited that are stored in the browser's cache.
When the virus tries to spread by sending out e-mail messages, it inserts one of the e-mail addresses it found as a false sender address. This makes it harder to find where the messages are coming from. Somewhere, someone's computer is infected with a virus. The virus found your e-mail address on their computer and is inserting it as the false sender's address. When an e-mail system receives one of those e-mail messages and it finds a virus, or the message can't be delivered for one reason or another, the system automatically sends a notice to the sender address. Since your e-mail address has been inserted (spoofed) you get the error messages instead of the actual sender.
I maintain part of the website at our university and have a link to my e-mail on all pages I handle. As a result, I am receiving several hundred messages with viruses and about one hundred bogus error messages each day.
Delete the error messages, and DO NOT open any attachments they may have. Make sure your anti-virus software is updated. If you use Windows XP, NT or 2000, make sure you have all the current updates installed. There is a recent update specifically for protecting computers from the recent viruses that spread without using e-mail..................Steve