According to the DBI book (the O'Reilly cheetah), the Sybase DBD driver offers some (limited) support for MS-SQL 7 servers with a patch (see related info here). You might also want to check if DBD::FreeTDS is mature enough for your uses yet.

You said you want to use DBI, but if nothing else works, there's a set of non-CPANed modules called MSSQL::DBlib and MSSQL::Sqllib available here. These don't use DBI and don't claim support for MS-SQL 7, so I'd shy away from those unless really necessary.