How can I make Windows XP bootable again?

You can use the instructions provided on ntldrmissing.com.

1. Get a blank floppy/cd-r/usb (whatever is on it will be erased), and put it into a working computer.

2. If you have a working Windows machine download fix: Floppy fixntldr.exe. For CD-R fixntldriso.zip and for USB ntldrusb.zip onto a working computer (Backup copies of these files available at their site).

3. Make the new boot disk:

Floppy: Don't put the fixntldr.exe on your floppy drive, put fixntldr.exe on your desktop or anywhere on your C: drive. 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. (If you don't like running an exe from me, you can run fixntldr_RawRite.zip and just replace the RawRite.exe file inside with one you trust) (now go to step 4).

CD-R: Don't burn the file fixntldriso.zip onto the CD. You want to first unzip or "extract" the iso file onto the desktop, you do this by right clicking fixntldriso.zip and choose "Extract All" then Next>Next>Finish.

Don't burn the file fixntldr.iso to the CD either, we will "extract" files out of it and burn those files to the CD via a special process depending on what OS version your working computer is.
Windows 7: Double click fixntldr.iso and follow the prompts to burn the contents to CD, then go to step 4.
Windows XP: Download and Install "ISO Recorder" then right click the fixntldr.iso file and choose "Copy image to CD" and follow the prompts to burn the contents to CD, then go to step 4.
USB: Come back and do step 4 after following the Screenshots of every step of making the USB disk and boot with it.

4. Put the new floppy/cd-r/usb you have just created into the computer that gets the NTLDR is missing error message, turn the broken computer off.

5. Power on the broken computer with the floppy/cd-r/usb inside it/plugged into it (if the computer was already on, restart it). Once your computer gets past the BIOS screen your computer should try to access the boot disk and you should see a black screen with white letters that says:

1ST TRY THIS seleccione esto primero
2ND TRY THIS essayez ceci en deuxieme
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

(The author 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:

Error 1. Windows could not start because file system32hal.dll was missing or corrupt

Error 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.

Error 3. I/O Error accessing boot sector file multi(0)disk(0)fdisk(0)BOOTSECT.DOS

Error 4. Immediate reboot

If you did get an error, try option 2. Most computers will boot with either option 1 or option 2. Of course if option 2 doesn't work, try option 3, etc, until you have tried all 10.

If none work, you can try pressing F8 at the "1st Try This" selection screen, it will give you a prompt where you can select Safe Mode, and then try the "1st Try This" option again.

If all goes right you will get your windows back. As I got on the first step very easily form this tutorial.