When you attempt to do a fresh (new) install of Exchange Server, Exchange Setup searches for installed components?
You can prevent this by using Regedt32 to navigate to:
In the right hand pane, you will see Value Names and data values that approximate:
AdminDest: REG_SZ : C:\EXCHSRVR\BIN MSMailConnector: REG_SZ : C:\EXCHSRVR Internet: REG_SZ : C:\EXCHSRVR Services: REG_SZ : C:\EXCHSRVR X400: REG_SZ : C:\EXCHSRVRIf Exhange Server is running, Use Control Panel / Services to Stop the Exchange services. Delete the Exchange folder (defaults as C:\EXCHSRVR), and delete the Setup key.