NTLDR is missing Press Ctrl+Alt+Del to restart booting problem

In Windows XP, If you  accidentally marked a partition on your hard drive as active ,and which does
not contain the Windows boot files then  you may not boot properly after that.And you may see this error message:" NTLDR is missing Press Ctrl+Alt+Del to restart "  .You can override this by following methods .Try changing the active partition by booting via a floppy disk and then using disk utilities to manually change the active partition: boot to a command prompt using a Microsoft Windows 9x / ME boot floppy disk.
At the prompt, type fdisk and press [Enter]. When you are prompted to enable large disk support,
click Yes. Click Set active partition, press the number of the partition that you want to make active,
and then press [Enter]. Press [Esc].
Remove the boot floppy disk and restart the computer.
If you don’t have a floppy drive, use Parted Magic it gives you a graphical interface to do the same thing.