When you try installing a new version of Microsoft Exchange Server and you have another version already installed or you were failed in installing any version of Exchange Server. You may get the following error:
"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".
Press windows key and R to Open Run.
Type regedit and then hit enter key
Go to following location
Rename Exchange key to _Exchange and then run the setup again.