Add thousand separator (SQL Server 2005)

-- seq table: 1,2,3,4,5....

declare @s varchar(100)
set @s = '3470040796'
print @s

select reverse((select substring(reverse(@s), [id], 1) +
 case when [id] % 3 = 0 then ',' else '' end + ''
from seq
where [id] <= len(@s)
for xml path('')))

--3,470,040,796