I would like to emphasis that SQL 2005 is another product versus just another version of SQL 2000. On the surface it may seem some of the features have disappeared in the product, but in actuality, it provides the DBAs with more flexibility.  For instance, there is an extended stored procedure facility that allows DBAs to write his own sprocs that would enhance the current xp_fixeddrives routine currently provided by today's version of SQL.  SQL 2005 also has the additional benefit of allowing the programmer or DBA wrote routines using VB.NET or C#.

In case to replace xp_fixeddrives just use scripting runtime library class Drive, which has property FreeSpace.