I have a Windows XP virtual machine on my Windows 7 x64 Ultimate machine that I use for testing. Every once in a while when I try and start the vm, nothing happens and I know I have to reboot my machine before I can use the vm. However, when I try and restart, my Windows 7 stays at "Logging off" forever. I've tried to kill the vpc.exe program before rebooting but I can't kill it. Nothing happens in task manager when I try and kill it. I let the machine sit at "logging off" for a while till I don't see much drive activity and then I force the machine off. I do have full admin rights on my machine.
If you have User Account Control (UAC) enabled, have you tried clicking in Windows Task Manager and killing the process from there?
Otherwise, open an elevated command prompt (Press Start, type in