@ Madman271
I'm using Yahoo mail, and it provides an excellent spam filtering; I'm also getting tons of sobig's every day, but they all end up in bulk folder, which means that those mails are obviously spam, if not dangerous. There's also a quick online virus scan available, just to scan suspicious mail, if you're not sure. Believe me, protection is much better than for average Outlook user. Infact, only virus which I got in 10 years is blaster, recently, which doesn't use mailing for spreading afterall.
@ Todays_Horse
Sobig scan address book on ifected machines and chose one address to send itself, and another to use as a fake origin address. In your case, virus used your address as a fake origin address, but it's not your PC that is infected obviously.
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Bojan Tarticchio