Sometimes you may get a message that says "Sys config file corrupt or missing", or something of that nature. Even if you don't get any message at all, many times the reason an XP machine won't boot is because the config file is corrupt. Here are some steps you can do to fix it. It requires that you have System Restore enabled, or at least had it enabled at some point in time in the past.
First, you need either the Windows XP CD or a bootable PXE disc like MiniPE. You can find MiniPE in most torrent sites. Using either of those, get to the command line and follow these steps...
Here are some notes before we continue. What you are about to do is basically a minor System Restore via the Recovery Console. I say minor because you're only going to copy part of the registry, not the whole thing. Since you are copying over a registry backup to be used you may loose some program(s) or their settings, it all depends on when the last Restore Point for System Restore was created. If some programs don't work it's really as simple as reinstalling them.
The text in red will be what you type and the black text will what the system outputs or what is put there by the system. READY, SET, GO.
C:\Windows>CD C:\system~1\_resto~1
C:\system~1\_resto~1\>dir
After you enter the DIR command and hit enter you will get a list of folders like such rp1, rp2, rp3, rp4, etc. etc. The "rp" part of the folder name denoting it's a Restore Point
What you'll want to do is find the 2nd highest numbered folder in that list. So, for instance, if it listed rp1 thru rp24 you'll want the rp23 folder. I'll use rp23 for the rest of this little fix just to keep things simple but substitute rp23 for whatever folder is the 2nd highest in the list on your pc.
Now, we have more commands..
C:\system~1\_resto~1>cd rp23
C:\system~1\_resto~1\rp23>cd snapshot
C:\system~1\_resto~1\rp23\snapshot>copy _registry_machine_system c:\windows\system32\config\system <<!!!MAKE SURE TO NOTE THE SPACES!!!
Overwrite system [y,n]: y
1 file copied
C:\system~1\_resto~1\rp23\snapshot>exit (before typing exit, please remove your Windows XP CD from the CD Drive)
After typing “exit” and pressing “enter” on the keyboard your computer will restart, hopefully into a working machine. If by some chance this did not fix your issue you can either go through this option again and try a different restore point or continue with the other options.
Hope this saves anyone from performing a complete wipe and install on a Windows XP box.