SymptomsYou downgraded from Parallels Desktop 8 to Parallels Desktop 7 and since then you cannot start your Virtual machine: it hangs on boot screen
CauseParallels Tools were not uninstalled prior to downgrading to Parallels Desktop 7 and Parallels paravirtualization driver from Parallels Desktop 8 conflicts with Parallels Desktop 7
1. Stop the Virtual machine
2. Go to Virtual machine menu ► Configure ►Hardware ►Boot Order
3. Insert the following string into the Boot flags field:
4. Start the Virtual machine
5. Go to Virtual machine menu ► Update Parallels Tools
6. Once Parallels Tools are successfully updated and your Virtual machine restarted, shut it down, go to Virtual machine menu ► Configure ►Hardware ►Boot Order and delete the Boot flag.