Same issue for me on a MacBook Pro.
I manually removed all things parallels except the VM in ~/Library/Parallels. Rebooted. Installed beta 5 and it launched fine.
Since I removed all prefs etc it required me to re-enter my key.
In hindsight I imagine this may have been solved by just rebooting and clearing the old statup items from memory. Or maybe by removing just the /Library/StartupItems/Parallels/ folder and rebooting before installing.
Last edited: Apr 27, 2006