Recently i posted on how to on Windows XP USB Edition – Boot Windows From USB Stick http://www.princeonlineservices.com/2011/07/windows-xp-usb-edition-boot-windows.html

i just found this trick interesting to share with guys How to Lock Windows XP Using Your USB Drive

If your work computer gets mobbed with people and you need to
stop others from using it here is nice trick that may help you.
What’s The Catch ?
Well the logic is simple. Whenever you PC starts it looks for the
boot device in BIOS. By default your hard disk is assigned as the
primary boot device. Once the device is found the computer looks
for the boot.ini file which contains the necessary boot information.
So if you can somehow remove/rename the boot.ini file and
replace it within your USB you can possibly use the USB as the
passkey to boot your PC.
Things you will need
1. This trick works for Windows XP.
2. An USB Drive.
3. A PC that supports USB boot.
4. A Windows XP installation disc (in case something goes
wrong). :|
Let us get started
1. Configuring Windows XP
At first we will be configuring Windows XP installed on your PC.
Follow the steps below and do exactly that is instructed.
1. Open Windows Explorer(Winkey + E) and go to your installation
directory i.e the partition where your Windows XP is installed. Go
to Tools > Options > View(Tab).
2. Now scroll down until you find the options as shown in the
screenshot below. Check/uncheck those as shown in the
screenshot. Once done, save the changes by pressing Apply and
then OK button.
3. As the hidden system files gets displayed, you will find the files
boot.ini
NTDETECT
ntldr
4. Now copy those files to a freshly formatted USB drive that you
will be using to lock your PC. Once the copying is done just
remove your USB drive.
5. Skip this step and go to the following section, “Configuring
Boot Priority in BIOS“. We will return here once again after we
are done configuring the BIOS.
6. Now rename the boot.ini file as boot.ini.bak. This will ensure a
backup of the original boot.ini file which can be used if you need
to revert back.
7. Proceed to the third and final section, “Test Drive“.
2. Configuring Boot Priority in BIOS
Restart your PC and press [F8] key to go to your BIOS settings.
Though for some PCs [F2], [F10] keys are also assigned for getting
into the BIOS settings. If you are not sure of the exact key, you
may try consulting your hardware manual which you got while
buying your PC.
CAUTION : Be sure about configuring your BIOS, if you are not
aware of what it does you may mess up everything. So please
consult someone who knows about it.
So now that you are reading this line, I will consider that you
know how to enter your BIOS and also know how to configure it.
Follow the steps afterwards.
1. Go to the Boot menu to change the Boot priority.
2. Assign your boot priority such that the primary boot device is
USB disk and the secondary boot device is hard disk.
3. Now save the changes and exit from the BIOS settings. In most
cases the [F10] key is assigned for the operation “Exit saving
changes”.
Okay now that we are all set, we will be restarting (without the
USB inserted) the PC and booting as usual. As told earlier return
back to step #6 of the previous section.
3. Test Drive
Step#1. Restart your machine without your USB drive inserted, if
you have done everything as instructed your PC will throw a boot
error. This means the boot.ini is properly renamed.
Step#2. Now insert your USB drive and restart(Ctrl+Alt+Del) your
machine again. Now you will see that, the PC boots completely
alright as it did earlier. This infers that the BIOS is set exactly and
detecting your USB device as the primary boot device and the
boot.ini file located within it is working properly.
Well that is all. Now every time you need to use your PC you will
have to use that USB drive and at the same time others who don’t
have the USB device will not be able to start your PC.
How to Revert Back
If at any point of time you need to revert back, just boot your Windows using the USB drive and then go to the Windows
installation drive and rename the boot.ini.bak file to boot.ini. There
is no need to change the BIOS boot priority settings. Now all
done, remove your USB drive and restart your PC without
inserting it.
What if You have Screwed Up everything ?
Well operating systems and hardwares are pretty notorious at
times and behave in an unexpected manner. It may happen that
though you have followed all the steps properly your PC doesn’t
boot at all. Well do not panic, you will just need to repair your
Windows XP once. click here to see How to repair  HOW TO REPAIR XP INSTALL