We have set up a VM that reboots every hour to provoke the problem. The machine happily rebooted for weeks, until new Windows updates were available. We configured the machine to create a complete memory dump and sent it to Microsoft for analysis. They suggested this fix
http://support.microsoft.com/kb/2591271
which points to problems with the IPv6 driver. After installation of the Hotfix, the problem has not shown again, although I think it is too early to call this case closed.