BartPE is like a live distrobution of Windows XP. It's based off of Windows PE (Preinstallation Enviroment), which was originally created to help computer manufacturers quickly and easily install software to finished machines.
BartPE is a like that, only it has its own little GUI and start menu that its creator wrote.
BartPE is awesome for spyware and virus removal from a 'clean' system. All you have to do is boot up, install your chosen software to the hard disk, and scan. It is an IT Professionals dream for those pesky virii and spyware that their customers or employers are faced with.
PE Builder is the software that creates BartPE. It copies its required files from your Windows XP/2003/2000 installation CDs to create its ISO. You can opt to add a directory to the CD with any utilities or software that you wish to install, which makes it very versitile. It can even write to CD instead of you having to use 3rd party software to do it. http://www.nu2.nu/pebuilder/
is the official BartPE webpage.