Error 1638: Another version of TurboTax is already installed
Error Code 1638: Another version of TurboTax is already installed can occur when you try to install another TurboTax "flavor" on top of an already existing one for the same tax year (for example, installing TurboTax 2011 Deluxe on a computer that already has TurboTax 2011 Basic on it).
Below is a screenshot of this error in the TurboTax 2009 version.
Only one TurboTax "flavor" per tax year can be installed on your computer, with one notable exception: TurboTax Business and an individual version of TurboTax can both co-exist on the same computer. (This is because TurboTax Business uses a different set of forms than 1040 individual versions.)
To get around this error, you'll need to remove (uninstall) the existing version before you can install the version you want.