Ip Spoof or Trojan?

I'm trying to decide if I've been infected with a trojan of some sort or if someone is just spoofing my e-mail account. I've been receiving dozens of failed e-mail messages - all spam relating to on-line medicine and that kind of chit - that are being returned to my e-mail account. The messages appear to have been sent at times when I wasn't even in the ouse, let alone on my PC. I've run every spyware scanner I can think of (Spybot, Ad-Aware, etc.) and I'm not turning up anything.

Any suggestions would be appreciated.