There are new viruses giving this message. This is a quick heads up to update your AV definitions, use a firewall and especially to run a windows update.
I'll edit this with new info later on, this probably won't be as bad as Blaster but is a similar vulnerability.
Timberlandko did a writeup where you can get help for the new Sasser worm (W32.Sasser.Worm) here: Sasser
The older Blaster worm that gave a similar NT Authority System message is dealt with here: Blaster removal