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This just happened to my wife's hotmail account the other day. It seems it was sending out e-mails to her contacts. I ended up running to different virus scans that took about 2 hours to complete and it turned up nothing.
I then just changed her password and her secret question and answer for her hotmail account and that worked perfectly. I would try doing that first.
Sounds like a newer worm, if so, some of the programs won't catch it right away.
I've been reading on several different forums that it's happened to others as well using hotmail, gmail, aol, etc..
I'm not sure if it made a difference but I started the comp in safe mode and then ran the scans.
Like I said, ever since I changed up her password and stuff there has been no problems.