All win7 machines on Mac host now give bsod after short time

[NotarySojac]

I've been using several Win7 virtualbox machines on our MacPro to run some Windows-only software. There has been no problem since I started. Within the past month all of them have begun bluescreening shortly after launch - anywhere from as soon as Win7 loads to an hour, but usually while I'm trying to do something like open the Event Manager, uninstall recent updates, or restore the system to a previous save point. When I can look at Event Viewer in a machine I can see a pile of Critical entries about a Kernel error that say basically "something happened".
The WinXP machine does not go kaboom like the Win7 machines do, so I suspect it's a particular Win7 update from Microsoft that is causing this.

Most of the time when I try to launch the Win7 boxes now I get the System Recovery prompt and when I try to restore from a previous save point it eventually fails. I don't think I get the bsod during Recovery. Only seems to happen when Win7 loads. It's happened several times while simply typing in my login credentials.
I built another Win7 machine from scratch last night and it was running fine until I let it suck down and install 110+ updates from Microsoft and now it's behaving the same as the others.

Has anyone else started encountering this since the start of the year?
I expect I will need to do another fresh Win7 build, then install updates one at a time until it starts giving the bsod.

VirtualBox 4.3.6 r91406
MacPro MacOS 10.6.8 8g RAM

[mpack]

Turn off Win7's "Automatic reset on system error" feature, and next time it happens you can report the BSOD error info.

You can provide the VM log file at the same time: Minimum information needed for assistance.