Can't install Parallels tools in v 5

I cannot install Parallels tools since upgrading to version 5.0 from version 4. I get the message in the lower left of the window saying that tools were not installed. Clicking on the gear to install or using the install option from the menu bar result in the same statement: "another instance of installer program is running or you do not have appropriate rights to run the installer. Please exit the other installer or contact your administrator."

If there is another instance of the installer running, why doesn't it install the tools? I have no administrator, as this is my home machine.

I have tried uninstalling and reinstalling the tools with the same result. I have powered down and rebooted.

I am running MACOS X 10.6.5 and Parallels build 5.0.9376 on a Mac Pro with Windows XP.