The following message can appear when installing TurboTax (similar messages may occur in older versions as well):
A version of TurboTax is already installed. You need to remove it before you can install this version. To remove TurboTax, use Add/Remove Programs on the Control Panel.
This happens when a previous TurboTax installation didn't finish installing or uninstalling.
To fix it, uninstall TurboTax and then reinstall it.