it should be current total
The you can simply say
Update tableNameHere Set Total = Credit - Debit
As I said before, you should look up triggers in SQL Server 2000 to see how you can associate the aforementioned statement with a table as a record gets updated.
I assume there is a valid reason for storing a calculated value in the table when a simple query would do. I suppose that's just part of the assignment, huh?