It worked for me on XP Home. If you're not comfortable with this whole procedure, DON"T DO IT! Bad stuff could happen to your good PC.
You're going to copy the "ntldr" file from a working installation of XP Pro and paste it onto the non-working hard drive's OS.
I'm going to assume both computers are old enough to have IDE cables (flat and wide) and PATA, not SATA drives, and a floppy drive on the good one. I'm not sure a thumb drive will work, but it might. I'm also assuming each PC has just one hard drive.
Remove the hard drive from the unplugged bad PC. Make sure you can put it back exactly the way it was - take notes if necessary. Both connectors only fit one way, and you don't want to damage anything trying to force them on the wrong way. And they're hard to get off, so grasp them by their 2 ends and sort of rock them end-to-end while pulling. Sometimes they'll have loops to pull on, which makes it easy. Don't pull on the cable, just the connector!
1.On the working PC, format a floppy disc and leave it in the drive.
2.Go to My Computer>Tools>Folder Options>View tab
Scroll down to "Hidden Files and Folders" - check "Show hidden files and folders"
Un-check the box beside "Hide protected operating system files (Recommended)"
3.Open Local Disk C:
Scroll down and find the "ntldr" file; right click>copy>"Back" button
4.Open Floppy drive A:
Right click>Paste>"Back" button
5.Copy and paste the file "NTDETECT.COM" the same way you did "ntldr" - it's just good to have a floppy around with both of those files on it.
*******Go back and re-hide everything you un-hid in step 2.********
6.Right click Local Disk C: and click "Properties".
Under the General tab, type "Orig" into the box at the top and click OK. Now the name of this drive will be "Orig C:" instead of "Local Disk C:" You can change it back when you're all done. This will avoid confusing which XP Pro installation to fix as you continue.
7.Shut down and unplug the computer, then open its case. You're going to put the bad hard drive into the good computer as a second HD.
Unplug the connectors from your CD/DVD drive and plug them into the bad hard drive. You don't have to put the HD in a slot - just don't have it hanging by the cables or have the underside where metal can touch the electronics - it will run fine upside down temporarily.
8.Plug the PC back in and turn it on.
This shouldn't happen if both PCs were set up the generic way originally, but if it tries to boot to the bad HD and says it can't find NTLDR, reply to my post and I'll get an e-mail that lets me know you need more help.
9.Go to My Computer. The good hard drive is named "Orig", so open the other one, as the drive letters might have been changed.
10.Repeat the un-hide operations from Step 2.
If there is a "ntldr" file there, right click it and rename it "ntldr.old"
11.Open the floppy disk and copy the "ntldr" file.
Right click the bad hard drive and click "paste".
Open the bad hard drive and make sure "ntldr" is there.
Make sure "NTDETECT.COM" is there, too. If not, repeat Steps 10 and 11 for it.
Re-hide everything (Step 2)
12.Shut down and unplug the PC. Remove the second HD and reconnect your CD/DVD drive. Plug the PC in and turn it on - it should boot as usual. Repeat Step 6 to rename the drive "Local Disk C:".
13. Reinstall the repaired HD into its unplugged PC. It should boot normally and be ready to use!
Best of luck, and thanks for what you're doing for your community!