If you've booted off that drive, you can't. That would destroy W2K.
Why do you want to do this? Unless we know your purpose, we can't advise you.
A reformat may be the wrong thing to do -- the rather uninformative way you
put your question indicates you don't know very much about W2K, which means
you don't really know what you're doing or planning to do, so I'm not going
to tell you how to do it. IOW, I won't give you unsafe advice.
Repost with more information, and you'll get help.
If you're asking how to format a drive you're going to
install Win2000 on, just boot off the Win2000 CD. It'll
give you the chance to repartition and format the drive
during the installation.
