Alternatively, boot up with the Win98 Floppy, and at the A: prompt, type "sys c:" without the quotes. This will wipe out the Win2K boot sector (which brings up the boot menu) and will boot your system directly to Win98. (Remove the floppy when you reboot, of course)
You'll have some files left in your C:\ folder that you can get rid of, too. AFTER you "sys c:" and verify that Win98 boots ok, go back into Win98 and delete the following files:
pagefile.sys
boot.ini
ntldr
ntdetect.com
and, if you have these files:
bootsect.dos
ntbootdd.sys
cmldr
These are all associated with Windows 2000. Additionally, you can delete the "Winnt" folder wherever you installed it.