Como instalar XP desde un USB

1) Formatted the USB flash drive with HP's utility, using Windows Enabler to make it bootable with DOS.
2) Slipstreamed an XP Pro CD using NLlite to include SP2 and the Intel Matrix Storage Manager (the SATA drivers)
3) Copied the slipstreamed i386 directory onto _both_ the USB flash drive, and the destination hard drive from the X61 (hooked up via external USB enclosure)
4) Copied smartdrv.exe, himem.sys and config.sys (enabling himem) from a Win98 boot disk onto the USB drive.
5) Boot the X61 from USB (Access IBM - F12 - select the appropriate USB drive), launch smartdrv, then launch winnt from the i386 directory.

At this point the install ran more or less normally. The one caveat is that you need to have the i386 directory on the internal drive too, because when XP is finally bootstrapping itself from the internal drive, it may not have access to the USB flash drive (as in my case).