If it is out of date then uninstall it and use AVG Free Edition click here. Widely used here...
"I have been getting a few mails back from people I dont know, rejected by their virus software. I think I may be infected. "
These MIGHT be the return from viruses sent out from ANOTHER PC but with YOUR address as the return address so whilst YOU are not infected someone else is and is sending out viruses in your name.
However you might also be infected.