Thank you,pete_4u2002.
I have installed SAV 10 on my servers for about 2-3 years .
It worked very well before.
The problem often occurs from 2011-3 to now,and I didn't do any change of my servers these months.
I think the problem is probably caused by the updates of SAV,but I'm not sure of it .
Now I'v added the KStackMinFree value in the windows registry.
But there are many system event(ID 1005) on the event viewer,like this:
Auto-Protect could not scan file C:\WINDOWS\FONTS\VGA936.FON for viruses due to low kernel stack.
Most of the files in the eventlog are under C:\WINDOWS\FONTS.
I don't know if it is harmful for system performance.