warning your system requires immediate virus scan?

I keep getting this message

 

warning your system requires immediate virus scan?

 

Then there is some other text after it...I always click cancel then it pops up a screen that appears to be doing a scan of my computer, but it is not the usual norton scan, it is a window with an address bar at the top. I close out...then nothing.

 

Any idea of what this is? I can't seem to pin point when this occurs. It happens really fast though. I've run system updates and virus scans with NIS 2009....nothing found, just tracking cookies that I remove.