So, here, you will learn how to make a usb-booting Windows XP installation. I have linux on my usb key, but you know that sometimes, having a fresh install of windows would be best. Now you can!

The problem: Your PC must support boot from USB.  Your usb key has to be a MINium of 256 mb and a MAX of 2 gigs.

First, you might want SP2 installed on the windows that will go on your usb key. And just maybe the gazillion patches. How do you do that? Slipstreaming!

Its a pretty length process, and to help, just go here

When you have your slipstreamed copy of windows,  you need to prep your usb key by using an HP format tool, along with BartPE
Using the HP format tool, format the drive using the FAT file system option. Next, you need the windows boot files. Go to C: in my computer. Go into tools>>folder options>>view>>select show hidden files+folders. deselect hide extensions, and the hide proetected operating system files. Press ok

In C: find boot.ini, NTLDR, and NTDETECT. copy those over to your usb key.

Open up PEBuilder. Select the location of your windows install files. Make sure Create ISO+burn to cd is deselected. Press build.

When it is done, open up command prompt. cd to C:\pebuilder313\plugin\peinst
stick in your usb key.
run PEINST.cmd through the command prompt.
See the nice black and white menu?
Type 1, enter.
Type the path to your build: probably C:\pebuilder313\BartPE
Type 2, enter
Type the drive letter of your usb key for the destination
Type 5, enter.
Watch your usb blink its happy little led mind off.

Installing programs is slightly harder, and I recommend you go here, and scroll to the middle of the article. Have fun!

Ok. Assuming you know how to make your computer boot from usb so its priority is higher than HDD, do it.