Well, the funny thing about SMTP is that anyone can write a message and say its from anyone. So you might just be the unlucky target of a spam ad campaign. Take a look at this article from Wikipedia on Email Spoofing. It gives a good description of what might be occurring and I won't have to repeat it here ;)
If you can check the header information of the emails people are suspecting come from your email account, you might be able to verify that they actually came from your computer or not (compare them to the headers of email you send legitimately). If they match, your system is likely compromised and the only way I would accept this as being resolved is to backup your data and reinstall from a known good source (like the Windows CD). Wipe the drives clean with DBAN maybe (use the 3-pass option).