Manually uninstalling Exchange Server 2007

Hi Experts,
After installing Microsoft Server 2008, I started to install Microsoft Exchange Server 2007. However, my installation media was damaged and the installation failed due to the fact that it was not able to copy all installation files. I tried uninstalling Microsoft Exchange 2007 from the Add/Remove programs but was unable to do so.
I keep getting an error that i should delete all mailboxes before proceeding with the reinstallation.
I did not create any user mailboxes, so i do not know what to delete.
I would like to know what the steps are to manually delete Microsoft Exchange Server 2007.
Which registry entries should be deleted?
Awaiting your answers.
Thanks!
