IE6 crashing repeatedly in ntdll

[chrised01]

2 machines, both running W2k, all current updates/patches except malicious software tool. AVG + Zone Alarm + Spybot.
IE6 crashes every time I visit about 25% of sites/pages, including the MS error reporting survey site! (click on Sign Up... crunch)
Error report shows access violation in ntdll.dll
MS crash site just suggests that I'm missing a critical patch - but it's only their malicious s/w tool that it lists as missing.
This only started happening about 2 weeks ago and affected both machines about the same time - I suspect a particular MS patch may be the cause, as the systems had no other changes in common during the same period.

Tried removing & reinstalling IE6 + patches on one machine - no better.
Swept machines with AVG, Spybot - no problems.
Tried Hijackthis - nothing obvious here either.
Only browser toolbar is Google - that's been installed and stable for over a year.
Is anyone else having a similar problem? 4 other machines running XP SP2 on the same network are fine.

Any suggestions much appreciated.
Chris