I think you're going to be pretty hard pushed to find much choice when it comes to free SQL Server 7 JDBC drivers.
Microsoft's driver only works with SQL Server 2000 - I believe that the only free driver that will connect to SQL Server 7 is FreeTDS - www.freetds.org
Check out this page which has an evaluation of SQL Server 7 drivers - It is out of date, but still contains some pertinent information.
Check out jtds.sourceforge.net, it supports SQL server 6 and 7 I believe. It isnt fully featured but its actively being worked on, and good enough for many simple projects. Its a full java implementation based on freetds work.