There are a few things to keep in mind.

Save the hidden files boot.ini, ntldr, and ntdetect.com. This makes it easier to restore the Win-2k bootloader later.

If your NT4 is on an NTFS partition, DO NOT BOOT TO WINDOWS-2000 until you have reinstalled NT4 AND reapplied a service pack of 4 or higher.

The C: drive cannot be FAT-32. If it is, use Partition Magic or the equivalent to convert it to FAT-16.

You will have to reformat any NTFS partitions containing NT4 because it cannot handle NTFS-5 partitions and interprets them as damaged.

After you have finished NT4 installation and the service pack (it is not necessary to reinstall all your software), boot to NT4 (if you're not there already) and overwrite boot.ini, ntldr and ntdetect.com with your saved files from Windows-2000. When you reboot, you'll get your Win-2k boot capability back. This is easier than running a repair disk.