I may have infected my computer with a trojan. Help me get rid of it.

I absent mindedly opened an e-mail from the "United States Post Office" claiming that a package I had sent on the 12th had an incorrect address. Then I clicked on the attachment and that's when I realized I had done a very bad, stupid thing because the USPS doesn't have my e-mail for any reason. Yahoo's e-mail did a scan and it said the attachment was clean. Google tells me that this is a trojan. I'm running Malwarebytes as I type. What else should I do? [more inside]
posted by Constance Mirabella on Nov 18, 2011 - 14 answers