First, be very careful when editing your registry. You can do serious harm to your operating system if you make a mistake.
Start > Run > Regedit
Drill down to:HKLM > SOFTWARE > Blackbaud andHKLM > SOFTWARE > ODBC > ODBC.INI
Find the entries that point to your database(s). Under ...Blackbaud above, our relevant ones are Raisers Edge > 7.0 and The Financial Edge > 7.0. Under ...ODBC.INI above, ours are AFNINI_1 and RE7_1. Make necessary edits there.
Again, be very careful. I'd strongly suggest backing up those registry keys before making changes so that you can restore if necessary.
-Sean