I have recently purchased an external USB/firewire external hard drive (Maxtor).
I had yesterday a failed shutdown of my laptop.
Since then everytime I reboot, checkdisk checks this external hard
drive. I have cancel the checking by hitting a keyboard key and it is
still doing the same following every reboot.
First, why XP is doing this? Is the new external hard drive faulted?
Secondly, how can I prevent checkdisk from continuing performing this action?

See the following document at Microsoft support:
"Chkdsk.exe or Autochk.exe starts when you try to shut down or restart
your computer":
http://support.microsoft.com/default.aspx?scid=kb;EN-US;831426

If this answer your question, please let me know in order to post this
comment in the answer box and claim the prize, if not let me know also
posting additional info and I will continue the research.

Regards.
livioflores-ga

Sorry,
but I went through this before i posted my question. Unfortunately,
did not solve my problem
1- chkdsk and autochk are not included in my scheduled task
2- the registry shows already autocheck autochk* and 
3- I am, as we speak,running error checking

You wrote: "I have cancel the checking by hitting a keyboard key and it is
still doing the same following every reboot."

My computer running xp does this whenever there is a unwanted
shutdown. Just let xp check the disk and complete it's testing.  Once
xp is done then your computer will start normally.

Try this:

After booting and when chkdsk is done checking the drive (after a few
minutes or however long it takes), open a command window (Start--Run
and type CMD).

Close all applications.

Change directory to the USB/Firewire drive.  (in case it is F, cd f: )

Type /chkdsk /f   and answer Y (yes) to the question on dismounting the drive.

Let Windows check the disk, after it completes EXIT the command
window, shut down the computer and shut down the hard drive.  Give it
a minute or two, then restart the hard drive and restart the computer.
 Let us know if it worked.

Thanks for the advice, but I decided to reformat the external HD,
which eventualy solve the issue. CHKDSK is not checking the HD upon
reboot