Possible solution
I've been running all versions of the Mac Pro "compatible" builds with few issues up until build 1920 which failed to launch like many others. Reboots, uninstalls then reinstalls etc etc failed to stop the crashing. I've followed all suggestions without luck.
So, I decided to remove everything and see if it fared any better. I ended up uninstalling Parallels (using the unistaller from the version that was currently installed), manually removing…
/Library/Parallels
~/.parallels_settings
~/Library/Preferences/com.parallels.*
… in Terminal, deleted the .pvs file I was using, deleted any used shared folders, rebooted, repaired disk permissions before attempting another installation.
Well… it now works. 
I do however still have the VT-x problem which sort of stinks. Oh well… can't have everything I guess.
EDIT : To delete these files in Terminal, obviously open Terminal and copy and paste the following line…
sudo rm -R /Library/Parallels ; rm ~/.parallels_settings ; rm ~/Library/Preferences/com.parallels.*
… then hit return. You should be asked to enter your password so do so.
Last edited: Sep 28, 2006