If you get "Error 1638" when installing Bentley Map, verify if you have another version installed on the machine.
Error 1638 description : Another version of this product is already installed. Installation of this version cannot continue. To configure or remove the existing version of this product, use Add/Remove Programs on the Control Panel.
Notice that the following Error 1638 is often a Windows generic error, so the description can be apply to other applications as well.