The prior KB915597 Windows Defender Update, dated April 22, was for Definition Set 22.214.171.124.
The latest KB915597, for Definition Set 126.96.36.199, is the seventh, and most likely last, for April 2010. 1.81.660.0 is a ‘BIG’ one — i.e., claims to fix a long list of ‘severe’ threats.
You can see the list of threats addressed by this update and others at this Microsoft site … click <here>.
1.81.660.0 has already been superseded. There are three new Definition Sets, the latest being 1.81.729.0. So we will most likely get another KB915597 this weekend or Monday to get us started up for May 2010.
I installed today’s update along with KBKB980408 — the latest stability and reliability update. I had to restart. All is well from what I can see.
Good luck. I will try to keep you posted.