The prior KB915597 Windows Defender Update, May 4, 2010, was for Definition Set 1.81.874.0.
The latest KB915597, for Definition Set 1.81.1622.0, is the second for May 2010 — and was dated May 12 though I didn’t install it until today, viz. May 14.
We had seven Windows Defender updates in April 2010, three by the fourteenth of the month.
You can see the list of threats addressed by this update and others at this Microsoft site … click <here>.
1.81.1622.0 has already been superseded. There are four new Definition Sets, the latest being 1.81.17699.0. So we will most likely get another KB915597 early next week.
I installed KB915597 and did a restart. That was fine.
Then went to do a scan using Windows Defender. Told me that the service was stopped and that my computer was not protected! Strange hadn’t seen that before. Obviously I didn’t stop it. Anyway, restarting only required a click of a button. Came up no problem. Didn’t have the time for a full scan. Did a quick scan. Took about 2 minutes. All appears to be fine.
Good luck. I will try to keep you posted.