There are many methods out there to create your own bootable Windows pen drive.the method shown is the easiest one.
To begin with you need a windows installation CD,DVD.You need atleast 2gb pen drive for Windows XP,and 4-8gb pen drive for Windows Vista, Windows 7

Step 1: First of download wintoflash tool and install it .
Step 2: Click the 'Windows Setup transfer wizard' and select 'Next'.

Step 3: Here you will need to define two paths where the 'Windows File Path' points to the location of the Windows installation disk and the 'USB Drive' specifies the location of pen drive  
that will be used for creating the bootable installation routine. Click on 'Select' and choose an appropriate path for both.

Step 4: Accept the 'Windows License Agreement' and click on continue.

Step 5: Don’t panic! The 'Warning' is just to remind you that the pen drive will be formatted before proceeding. Click 'OK' to proceed or 'Cancel' in case you inserted the wrong pen drive or want to check it.

That’s all that needs to be done to be able to create a bootable Windows installation pen drive. To be able to boot from the pendrive you will need to access the BIOS and change the boot priority to 'Removable disk'. If the settings are right you will be able to boot and proceed with the installation of Windows...