Microsoft has released, via Windows Update, a newer version of the Malicious Software Removal Tool that finally finds and removes the Srizbi spam sending botnet code. Almost all spam is sent from compromised PCs like yours (and maybe you are actually infected!!), and this free Microsoft tool will find this malicious code on your PC and remove it.
So … maybe you should be absolutely certain to run Windows Update. For more details you can read the article on IT World.Share