Unfortunately I believe Microsoft's JDBC driver (available the location below) is only for SQL Server 2000.
For a free implementation of a type 4 JDBC driver for SQL Server, you should check out the FreeTDS driver:
A number of JBoss projects that talk to SQL Server databases use the i-net Software Opta driver - this isn't freeware, however there is an evaluation version that you can download and test. We've used their drivers in our systems for some time, and they're very stable. (and no - I'm not getting paid to say that!)