The table sysindexes has most of the information generated by the sp_spaceused. Querying sysindexes is probably easy, since you get space usage for the entire database at one shot. Iterating on the table is easy through cursor.

Here is a link to the MS article that explains how to interpret the values from sysindexes table.

http://support.microsoft.com/default.aspx?scid=http://support.microsoft.com:80/support/kb/articles/Q90/7/58.asp&NoWebContent=1

Let us know if that has been helpful.

Regards,