You want to upgrade to Parallels Desktop 9.
NOTE: After upgrading to Parallels Desktop 9 you do not need to reinstall Windows or create a new Virtual Machine.
You can successfully use your existing Virtual machine.
Download the Parallels Desktop 9 installation .dmg file and mount it to Finder. Double-click the Install icon.
Follow the prompt to remove the previous Parallels Desktop version:
Wait until the removing and installation process is over.
Upgrade Parallels Tools in each of your Virtual Machines: