Setting Duplicate Fields To NULL

I have a result set stored in a temporary table variable. The fields need to have heirarchy when they finaly get displayed to the user. I need to replace the repeating fields per group with NULL and leave out only one set of unique records per group.

I want to do something like this- make the fields in red NULL while leaving out the ones in green.

{ Col.Group I } { Col. Group II }

Col1 Col2 Col3 Col4 Col5
------- ------- ------- ------- -------
ValA ValA2 ValA3 ValA40 ValA50 {
ValA ValA2 ValA3 ValA40 ValA50 Row. Group A
ValA ValA2 ValA3 ValA41 ValA51 }

ValB ValB2 ValB3 ValB40 ValA50 { Row. Group B
ValB ValB2 ValB3 ValB41 ValA51 }



I was able to achieve this by using @@ROWCOUNT with in a while loop by performing one UPDATE at a time (SET ROWCOUNT = 1). But i have to run Count(*) each time I loop through the update query to figure out the remaining number of rows I have per group as I need to leave out one untouched. But the Count function is taking too much time and I'm having performace issues.

Any idea how I might be able to do this in my stored proc. efficiently?

Thanks,

Samtadsh