How To Put Serial Numbers For Results In Group

I need to put the serial numbers for results in group in SQL Server 2000. Please see below:

My input:
procedureid procname
1 A
1 B
2 A
2 B
2 C
2 D
3 A
3 B
3 C

Output I need:
procedureid procname serial_num
1 A 1
1 B 2
2 A 1
2 B 2
2 C 3
2 D 4
3 A 1
3 B 2
3 C 3

Here is my table:

create table po(
procedureid int,
procname varchar(10),
)
insert into po values (1,'A')
insert into po values (1,'B')

[Code] ....