SQL Server 2000: sequential number

Hi,
I've a SQL Server query like this:


select a.id, a.desc_id, b.id_num
from tab_a a, tab_b b
where a.id=b.id

I need add a row number at this query, that returns the sequential number of a row starting at 1 for the first row.

In SQL Server 2005 I know that exists ROW_NUMBER function but I need a similar function in SQL Server 2000.

How Can I add a sequential number (like identity) in my query?

Thanks in advance!