Can not uninstall a software program

The Charity that I volunteer at is trying to remove their current corrupted Corel software and than reinstall the software.

Their software could not be removed by using “Add Remove Programs" as they keep receiving error messages. I went to their registry and removed all references to their Corel program and rebooted their machine.

We tried installing their software from their CD and keep receiving this error message, "Another Version of this product is already installed".

How can we fix this error message and reinstall their software?

Thank you for your suggestions