If you get
HP's official WM2003SE 1.1 ROM update, then there should be an .nbf file in there. That file is what you need.
You then need
this bootloader.
Unzip the bootloader to a directory without spaces (I just use "C:\Bootloader\").
Then, run the HP ROM update. It may flash your unit, it may not. If it doesn't, it should leave behind the temporary files in the C:\iPAQ\SP29841 directory. In there is the .nbf file you need, and perhaps an .nb0 as well. Either will work.
Now, go to the Bootloader folder and run the executable. When you get a browser window popup, point it to the .nbf or .nb0 file, and it'll take care of the rest.
Of course, this all assumes that the PC doing the ROM flashing is running Windows XP. I've never had it work on Vista-at least not 64-bit Vista, anyway. You may have better luck on a 32-bit Vista installation in that regard.