Going from WIN 98 to WIN 2000 or XP PRO

I have windows 98 currently on my computer. I have tried several times to install either windows 2000 and even XP. It just wont work. I tried installing the new windows over 98. I would like to try a clean install, but a message pops up saying that setup cannot run in DOS mode. How do I get around this in order to do a FRESH install?