1.
System Changes.
a. Have you made any hardware or software changes recently?
b. Have you modified the BIOS settings recently?
c. Has your system clock been losing time or displaying wrong date-time-group?
2.
Boot hard drive. Does it physically use IDE or SATA interface?
3.
Operating System. What operating system and service pack level is Gateway system currently at; e.g. Windows XP Home Edition SP3?
4.
chkdsk. On general principle, I would boot to Windows XP Recovery Console and execute the "chkdsk /p /r" command; in the hopes it would repair any file corruption. Don't interrupt it, let it run to completion. Then enter "exit" command to permit system to restart.
If that doesn't fix it, re-enter Recovery Console and execute the "fixboot" command, then "exit" to permit system to restart.
5.
Recovery Console.
a. You may download and create five (or is it six?) floppy disk volume set that will permit you to boot into recovery console.
b. You may download and "write" (not copy) bootable Recovery Console CD-R.
c. You may borrow a Windows XP setup CD. Not sure what version or service pack level it must be.
6. Reference:
Stop 0x000000ED (UNMOUNTABLE_BOOT_VOLUME)