Short of reformatting and starting over, you can't. All the XP boot
information is contained on the C drive. If you format or otherwise remove
Windows 98 from that drive, you'll lose that information and the ability to
boot the system without starting over from scratch. This assumes XP created
the boot menu during XP setup. If you used third party software such as
Partition Magic or Bootit it can be done but if you had used third party
software to create the dual boot setup, my guess is you wouldn't be asking
the question so the bottom line is, unless you are prepared to start over
from scratch and reinstall XP, you can't.