Just got done doing a pretty large website using MSSQL on WINNT 4.0 with IIS. Unfortunately I couldn't get the MSSQL drivers to connect so I had to use ODBC. It connected great.
First you have to setup the ODBC data source on the system you are running IIS. Make sure you test the connection.
Also make sure your PHP.ini file has ODBC support enabled. This is where my problems were at first.