I've had to fix three computers this weekend so far with the SoBig (none were mine, just folks I work with). They swear they didn't open any attachments, but they somehow had the virus. I'm pretty sure they opened one. Only way to get it.
What you are describing is exactly what they had and it turned out, they did have the little booger.
You might want to run this tool just to be on the safe side. Can't hurt.