The prior KB915597 Windows Defender Update, dated April 30, was for Definition Set 1.81.660.0.
The latest KB915597, for Definition Set 1.81.874.0, is the first for May 2010 — and was dated May 3 though I didn’t install it until today, viz. May 4.
We had seven Windows Defender updates in April 2010.
You can see the list of threats addressed by this update and others at this Microsoft site … click <here>.
1.81.874.0 has already been superseded. There are three new Definition Sets, the latest being 1.81.949.0. So we will most likely get another KB915597 by the coming weekend or Monday.
I installed KB915597 and did a restart. Then did a full scan using Windows Defender. All appears to be fine.
Good luck. I will try to keep you posted.