Is there an ODBC driver that can connect our existing Informix database on Unix to SQL Server 7.0 on NT 4.0?
Yes. You need to run the Informix I-Connect product on the NT system. Informix databases are ODBC compliant, so once you get the I-Connect running you should be able to access Informix from SQL Server. If you need to go the other way, you'll need to get the ODBC drivers for your flavor of Unix. Call Informix.