The good news is that your computer is probably not infected. The reason that you get these messages is that viruses seek email addresses in the infected computers directory and uses them when they send out new infected email messages. It makes it harder to track where the infected email came from. So your email address was likely in a directory on someone else's infected computer. Your computer is probably OK.

The bad news is if that you are not sure that your computer sent these out, you are not as well protected as you should be.

I use Norton Anti-Virus. It scans everything coming into the computer, emails included. It also scans outgoing emails. So it is very difficult for viruses to get in or out of my computer. NAV also checks for updates to the virus definition file every four hours and at start up. If there are updates, they are automatically downloaded and installed. You can never be too safe. I have not been infected since using NAV many years ago. I get virus infected emails daily and NAV deletes them before they do any damage.