Chances are you already either have a USB "thumb" drive, or you will have one in the near future--these little solid-state flash memory devices are inexpensive, nearly ubiquitous, and very, very handy. Not only can they replace floppy drives for casual file transfers, but the larger capacity thumb drives also can serve as the basis for an excellent, fit-in-your-pocket software repair kit, letting you diagnose and repair PCs, including those that might otherwise be unbootable or that are locked by password or other problems.
We've discussed USB drives before in "What's Behind The USB Drive Revolution". We also looked at ways to make a USB drive bootable from DOS in "Solving USB Boot Problems". We've also mentioned ways to boot from Linux (such as is discussed in this HowTo or in this Google search.) If you missed either of those earlier articles and references, this would be a good time to click back and catch up so we'll all be starting on the same page.
But when we left off our discussion of USB drives, there was no good way to boot XP from a thumb drive because of the way that XP takes over USB control as the operating system starts -- in effect, XP tries to take over the USB system it's already using to boot from, with unpredictable results.
Since our earlier discussions, three things have happened that can make it much, much easier to get XP to boot from a USB device. First, Microsoft released new software that can manage the USB handover much more smoothly. The new software is part of Service Pack 1 for Windows Server 2003, but can work fine in booting XP and even Win2K from USB. (We'll tell you how to get free copies of the new software in a moment.)
The second piece involved Bart Lagerweij and his justly famous "BartPE" software: The BartPE software is a free and powerful way to create custom bootable, self-contained XP CDs that are ready-to-run and that come with a full battery of software tools for PC diagnosis and repair. The BartPE XP CDs are like the "live" versions of Linux -- everything runs right from the CD. (See "A Must-Have Repair And Recovery Tool").
Bart produced a new USB installer for his tool, making use of the files that came in SP1 for Server 2003. The installer is actually a highly-automated series of scripts that can build a self-contained USB-bootable version of the BartPE XP repair and recovery environment. This gives you what's arguably the most powerful recovery and repair tool ever -- and a self-contained version of XP -- that can fit on, boot from, and run on a modest 256MB USB thumb drive!
The final piece fell into place when some other software authors helped resolve some of the most common remaining issues, so that XP-boot-from-USB now can work in many, many cases.
What Works, What Doesn't
Before we get too far, it's worth noting that there are some kinds of boot-from-USB problems that software fixes can't cure. For example, not all PCs (especially older ones) support boot-from-USB in the first place: If the system doesn't allow it, it's game over. (If your PC supports boot-from USB, it'll be an option in the BIOS setup tool.) There can be compatibility issues with USB 1.0 and 2.0 hardware, and with "High Speed" versus "Full Speed" or "Low Speed" support. And so on.
More subtly, some systems that support boot-from-USB may do so either via hard drive emulation or floppy drive emulation by the USB device; but usually not both. If you try to use a USB drive that's emulating a hard drive, but your system only supports floppy-drive emulation (or vice versa), you may be out of luck. Alas, many PCs aren't labeled as to which forms of USB boot emulation they support, leaving you to experiment. (More on this in a moment.)
And there also can be issues in the USB devices themselves. For a list of many USB drives that are known to work with Bart Lagerweij's tools -- as well as many known not to work, see post #17 in this thread.
Yes, it's a little confusing, but don't worry: If you're not sure that your PC and/or USB drive is suitable, you can simply go ahead and try the full XP boot method we'll discuss: If it fails, it most likely simply won't work. It should do no harm.
But if you want more surety, it may be worthwhile to try any of the simpler DOS boot tools mentioned in "Solving USB Boot Problems". A USB drive that can boot from DOS probably can be made to boot from XP. But a USB drive that won't even boot from DOS probably will be very difficult, at best, to get to work with XP. Thus, running a quick test with the DOS-boot tools can help you decide if it's worthwhile to try the longer, more complex XP boot method we'll now describe.