Cannot get beyond Vista Welcome screen

I have build 3846, and have had Vista running fine for a while when pointing the VM at Vista's boot camp partition. But After a spell of running Vista directly from Bootcamp and now trying to start it uip again from OSX, Parallels just hangs at the Welcome screen of Vista. I.e. UI have logged in.

Tools are installed (according to the icon).

Vista is completely unresponsive.

If I reboot the hardware and run Vista directly, it starts up OK. It just can't run under Parallels for some reason.

I had a similar problem months ago, and found an obscure reference in the docs to some magic key combination that would allow me to progress, but try as I might I cannot find that info again.

Any suggestions?