How can I find the total space used by a database in SQL Server 6.5?

I did the following after reading the documentation-
1) updated space usage info (using dbcc updateusage)
2) ran sp_spaceused for the database

The problem I am facing is that unused space doesn't seem to match the total free data space remaining. Also the reserved space reported is not equal to the total data space available for the database.

Can someone please help me.....