Well it's definately not a microsoft WGA notice, go to msconfig and post a screen shot of whats in there, see if we can't figure out what's causing this. How often does this happen? Is there anything specific you do that causes this?
*edit- also check your event viewer and see if anything sticks out. right click my computer>manage> expand event viewer and look under application, and system, double click on the events to see more specific info on them, post the results here