While running the Exchange Server Deployment Tool (ExDeploy.exe), you eventually arrive at the step of running Forest Prep of the Active Directory.

Insert Exchange 2003 installation CD into the CD drive. Open Windows Explorer and (optionally) browse to D:\Setup\i386\setup.exe and double click to run setup.exe. On the Exchange Deployment List there is a link below the browse input box. After you have selected the setup.exe file at the input box, Click the link to “Run Forest Prep Now.”

Depending of versions, you may see an alert box stating that Exhange has compatibility issues with this version of Windows. Just check OK Bypass it to do the install of forest prep. Click Finished.