Driver Problem
I had a similar but different problem. After the update, the Parallels Tools auto-started to install the new tools. The process halted at the Audio Driver, said it couldn't be installed, and the whole process rolled back to a state where NO drivers were installed on the system. All subsequent reboots resulted in the same failure.
The fix was to recover the .pmv file to an earlier state via Time Machine, and then change it's configuration before launching to one CPU, 1 Gb of RAM. The time rebooting after changing it back to 4 cpus and 2Gb of RAM resulted in a blue-screen, but subsequent reboots were OK.