Vista OS Stability.Anyone else have same problem?

Vista worked fine until 20th Feb 2008 until Windows installed a new Defender definition KB915597 1.27.6845.0. it crashed 6 times that day.
There followed a series of checks over the next 3 days while the 'Stability Graph' continued to fall. I then removed Zone Alarm. It's now some 20 working hours since that action and all appears well. The Stability Graph has also started to climb again.
I hope this may be of help to others.