Speaking about XP, isn't it? Because there is an old service pack, with a boot loader who can see just a few dozens of files in the root directory of the C: drive, and any file which exceeds this limit will not be seen at boot time, so, if ntldr is moved in such a position, (e.g. by a defragmentation operation), it will not be found after reboot, so you'll have to reinstall. The solution is to apply at least the Service Pack 2, as soon as installed, because this will update the boot loader code with a more powerfull one. Good luck!
