DISCLAIMER: I am not a SQL administrator and I have not done this migration. I have installed Rightfax on servers that had multiple SQL services on it, so this should work. I am going to perform this procedure in the upcoming weeks so if it blows up, I’ll update the post. 
1. Install SQL 2005
2. Use SQL manager (not sure of exact name. Your DB people should know) to backup rightfax database. Stop the Rightfax services before backing up the database.
3. Use SQL manager to restore rightfax database to SQL 2005
4. Reconfigure the ODBC connection to point to the new location.
1. Go to Enterprise Fax manager.
2. Double Click Rightfax Database Module
3. Click on the three dots under SQL Connection
4. Click the three dots in ODBC Data Source
5. Go to the System DSN tab
6. Configure Rightfax (or whatever naming convention was used)
7. Under SQL server you want to connect to, select the SQL 2005 server\Rightfax. You can find the path by using the SQL manager. It will prompt which server service to connect to.
8. Start the services.