I suggest you completely de-install the Parallels tools from your Windows VM (using the Windows uninstall feature), reboot the VM, and then re-install the Parallels tools. If that doesn't work, try uninstalling Parallels using the uninstaller that comes with Parallels - the uninstaller will ask you if you want to keep your VM files, say "yes" - rebooting your Mac, and re-install Parallels. Start up your VM and install the tools. Good luck!