iSight only shows black image after auto-update (WinXP, Snow Leopard, Parallels 4)

Problem description:
I have been using iSight within Windows for quite a while and it never failed,
but after some automatic updates a few weeks ago iSight does not work properly anymore under Windows XP (everything is fine in OS X).
It is recognized correctly and all system tests indicate no problem, even the green status LED is turned on in video mode.
But instead of showing the current video image, it just shows a black image.
I tested this in the Windows Explorer as well as in Skype with the same result.
I also used a different webcam but the result was exactly the same (camera was recognized but no image).
It looks like the video signal is not coming through anymore, for whatever reason.

Has anybody had this problem and knows how to solve it?
I can image three possible error sources:
Either the Windows update, an update to Mac OS or an update to Parallels.
My guess would be a Windows update, but I'm not sure about this.

VM: Parallels 4.0 (Build 4.0.3848)
OS: Mac OS X (Snow Leopard), Win XP (with SP 3) (both with auto-update enabled)
Computer: white Macbook (ca. 2-3 years old)
iSight driver: 2.0.1.0 (this is one of the versions I tried)
Tested solution:
I tried to follow:
http://kb.parallels.com/en/4895
but on my system there is no diskimage.dmg in the described folder.
Instead, I downloaded the iSightInstaller.exe from Apple
and I also tried the version that comes on the Snow Leopard disc.
Unfortunately this did not solve my problem.