I have similar behaviour. My win7home premium 64 bit guest was running fine under Ubuntu 11.10, but after upgrading to 12.04 yesterday it no longer works. Initially it would not start, giving an error message that kernel modules were not loaded, but after doing a dpkg-reconfigure for the dkms package and starting via inet.d the guest would begin to boot. Shortly after starting the machine freezes. Kbd no longer works, and mouse clicks have no effect, though the pointer still moves. Unfortunately I do not have any snapshots to try.
The issue is not just my guest install, I tried creating a new vm and installing win7hp-64 and the system freezes in the same way during the first graphical "starting" screen.
Any help would be appreciated.