I install NT workstation on my system. Due to some problems I repair NT workstaion. After that I can't able to reinstall NT service pack.(SP4). It say that the registry key is already exists. how can i troubleshoot this problem

When you apply a service pack you're given the option to SAVE the previous configuration. Hopefully, that's what you did. Uninstall Service Pack 4 and disable all non-essential services and devices before you reapply it. There's a cautionary paragraph or two of instructions that comes on the disk with the SP4 (or any service pack) CD that goes into detail about this. It sounds like you didn't even get far enough to finish applying the service pack. If the system is hung up, you should just go back and reboot and repair the system using the ERD per your previous repair procedure. Other than that, you might want to get acquainted with the troubleshooting wizards at: http://support.microsoft.com You can go there right now. For 1) select NT Workstation 4.0, for 2) select keyword search using exact words, for 3) type "registry key already exists." If you don't like any of the links you explore there, go to the bottom of the (next) page you're taken to when you click "go" and click on the troubleshooting wizard link.