After removing Exchange 2010 and trying to reinstall you get error Database is mandatory on UserMailbox. Property Name: DatabaseDecember 17, 2010
Resolving Winrm errors in Exchange 2010December 23, 2010
To first resolve this problem obviously you need a domain controller in the same site as the Exchange 2010 server you are trying to install. It must also be a global catalog. Both of these items can be configured via the active directory sites and services snap in.
If y0u have verified that simply use the command line to initiate your install point setup.com to the correct AD server.
setup.com /mode:install /role:mailbox /dc:DCinMYsite