This may be one of the great problem faced by windows users
What happened actually?
When your computer starts, the BIOS attempts to find the primary hard drive's active partition to read the first sector for the MBR (Master Boot Record), it uses that info to load the rest of the OS. For Windows NT4/2k/XP the NTLDR (New Technology Loader) takes it from there. If you get the "NTLDR is missing, press any key to restart" what's most likely going on is the BIOS either didn't look for the right drive, didn't find the right partition, it wasn't active, didn't find the MBR, or the MBR didn't list NTLDR in the right place, the location of NTLDR changed, or you are looking at a hardware failure situation (memory/cables/drive/motherboard/etc). Windows Vista does not boot this way, you can still use my floppy to boot into an existing installation of 98/nt/xp, but I've not had a chance to test Windows Vista.
The quick test to make sure your OS installation is still good is to create an MBR and NTLDR on a floppy disk and check your partitions, this disk will check many of the partitions for a working windows installation. Here are the instructions to do this:
1. Get a blank floppy (whatever is on it will be erased), and put it into a working computer
2. Download fixntldr.exe onto a working computer
3. Run the fixntldr.exe file by double clicking it. Click OK to overwrite the blank floppy disc in the working computer, you should see some screens about writing a new floppy disk
5. Put the new floppy (or cd/usb drive) you have just created into the computer that gets the NTLDR is missing error message, turn the broken computer off
Using the boot disk in the computer with the "NTLDR is missing" error.
Start back up the broken computer with the floppy in the floppy drive (or cd in cd drive or flash drive in usb port). Once your computer gets past the BIOS screen your computer should try to access the floppy drive and you should see a black screen with white letters that says:
1ST TRY THIS seleccione esto primero
2ND TRY THIS essayez ceci en deuzieme
3RD TRY THIS wahlen Sie diesen Third
4TH TRY THIS selezioni questo fourth
5TH TRY THIS selecione este fifth
6TH TRY THIS seleccione este sexto
7TH TRY THIS essayez ceci en septieme
8TH TRY THIS wahlen Sie dieses achte
9TH TRY THIS selezioni questo nono
10TH TRY THIS selecione este decimo
(I threw in some Spanish / French / German / Italian / Portuguese for international flavor.)
This file is set up to automatically select the "1ST TRY THIS" choice after 30 seconds. Try it first, if it was the wrong selection, you will likely get one of these four errors:
1. Windows could not start because file "
2. Windows could not start because of a computer disk hardware configuration problem. Could not read from the selected boot disk. Check boot path and disk hardware. Please check the Windows documentation about hardware disk configuration and your hardware reference manuals for additional information.
3. I/O Error accessing boot sector file multi(0)disk(0)fdisk(0)\BOOTSECT.DOS
4. Immediate reboot