Note : Scroll down to summary if you don't like to read long rant

Hell yea, there are still plenty computer running Windows XP this day.
So several days ago, one of my clients run their computers on small electric generator genset. Apparently this generator is faulty and suddenly it shut down without warning. To make the matter worse, some of the UPS are not functioning so the computers are dead instantly.

After they switched to new power source, as usual they switch the computer back on. As these computers were not shut down properly, Windows asked for they action users want to do (Safe Mode, Normal, Last Known Good, etc.)
Naturally the users choose "Last known good configuration" option.

While most of the computers are working normally after this point, two computers refuses to do so and keep rebooting after the Window logo appear for seconds on the screen. So the users then panicked, as dead line approaching and no backup available (typical). So they call me to see what can I do to help them.

First, I tried the "Safe Mode" to see if I can at least log in to the Windows. But the computers still rebooting after loading Mup.sys
From this clue, it appears to me that it's a hard disk problem.

So I grabbed one computer that works normal, and then I put the hard drive from the faulty computer as secondary drive on the working computer and then start the computer.
After I logged in to Windows, then I do chkdsk on the faulty drive from command prompt.

Its very easy, just make sure you know the correct drive letter of the faulty drive (in my case it was F:).
To go to command prompt: click Start and choose Run, then type cmd followed by enter button.
Once the black window of command promp appears, type chkdsk f: /f followed by enter button.
Windows then proceed to scan the drive and fix any error it encounters.

When the process finished, then I installed the faulty drive back to its computer unit. and viola! it wooorkkkssss!!!!

However, the other one is not that easy, its still doing the reboot loop after the disk scanned, so i repeat the process but this time i use the chkdsk f: /r command. This is the same as chkdks /f but it do additional task to try to recover data from bad sector, so it will take longer time than (in my case it was a bit more than one hour) but it worth the wait, because after this the computer is back to work normally.

I pointed out to my client that they need to do regular backup and install some good UPS to avoid the same problem to happen in the future.

If you have reboot loop after improper shut down on Windows XP computer, try the followings to try to recover:
1 Find another Windows XP computer that works normally.
2 Remove the hard drive from the faulty computer and install it on to the working computer as secondary drive (make sure both computers are switched off and disconnected from power source)
3 Switch the working computer on
4 After sucessfull login to Windows, find out the drive letter of the faulty computer (for example F:)
5 Go to command prompt by clicking Start>Run
6 Type in cmd and press enter button
7 On the black window of command prompt, type in chkdsk f: /f and press enter button (replace the f: with the correct drive letter), and wait until Windows finished fixing the drive.
8 Shut down the computer, and remove the faulty drive and install it back to its original computer unit.
9 Pray to your God (optional step)
10 Turn on the faulty computer and wait till it load Windows.
11 If you still have the reboot problem proceed to next steps.
12 Swear (optional step)
13 repeat from step 2, but on step 7 change the /f to /r so in this example the command will be chkdsk f: /r, the rest are the same.
14 So if you still have the reboot problem, then its time for harder tasks, maybe reinstall or even harddrive replacement.

So I hope this can be a help of you encounter the same situation.