Hi,

I had installed windows NT 2003 in the same drive (C:) in which an earlier version of windows NT 2003 is installed. How do I remove the newly installed OS and leave the original one intact.

You just need to remove the partition that NT is installed on. By creating the partition in the first place you have logically split 1 hard drive into 2, so you can alter them independently. There are a host of partition management apps out there, Google away...