How to format HD with Wn XP Pro on it?

I was told that my system should have a different Windows on it.
XP-Pro making it run to slow.
Want to load Win 98SE or Mil. in it.
So how can I format this HD?
Try everything I can think of.
When I get to end of formatting I get message.

YOU CAN NOT FORMAT A NETWORKING HD.

WHY NOT?

This is the only PC in the house, not connected to any networking.
It’s running Win-XP-Pro
I know there has to be a dos command that will let my get this out of my PC.
Hope someone can help?