Pat;
Windows XP does not have DOS, so you need a Windows 98 boot disk to
boot to DOS.
If you are referring to Blaster, follow this to get rid of it.
You most likely have a worm W32.Blaster.Worm
DISCONNECT the subject computer from any network IMMEDIATELY.
If necessary to stop the reboot process:
Start/Run
Type "shutdown -a" ENTER
VERY IMPORTANT to follow ALL steps, closing ports or installing the
patch is NOT enough.
Download the patch and updates referenced below.
You may need to do this at an uninfected computer and burn to CD.