install thinks VM is running, but its not

I'm attempting to install the build 11994 update to v6, but even though Parallels is not running, I keep getting a message indicating:

"To continue the installation, quit the following applications: Parallels Virtual Machine".

Is there a temporary file which tells Parallels that a VM is open which might have been improperly left behind if a shutdown went awry? If so, where would it be located, and does it just need to be deleted, or is there a settings file somewhere which needs to be edited?

thanks...Bob