@Greg
brkirch is not talking about installing Parallels on OS X, but rather the windows side of Parallels - referred to as Parallels tools.
This software is responsible for relaying in/output between the host and the VM and also for setting up and running the correct drivers for your virtualized hardware.
Seeing as the problem with your mouse is probably driver related - a reinstallation of the Parallels tools after a downgrade of Parallels makes perfect sense.
brkirch is not talking about installing Parallels on OS X, but rather the windows side of Parallels - referred to as Parallels tools.
This software is responsible for relaying in/output between the host and the VM and also for setting up and running the correct drivers for your virtualized hardware.
Seeing as the problem with your mouse is probably driver related - a reinstallation of the Parallels tools after a downgrade of Parallels makes perfect sense.