Hey Icecube, it was a while ago that I created the v4 one, but I believe this was the guide I followed. Trying to recreate it now and I've copied all of the files from the working v4 image...
1. Extract the ubcd files from the iso
2. Copy all the files from i386 off your xp setup cd into the root of the ubcd (ie /ubcd/i386)
(If you just want a bootable recovery console I downloaded one some time ago as an iso with just the files required to run it, it is less than 10mb total. Unfortunately I have no idea now where i got it then.)
3. Get Bart's Bart's wxp10.zip: http://www.nu2.nu/download.php?sFile=wxp10.zip
4. From wxp10.zip copy w2ksect.bin win51 and win51ip to the root of ubcd
5. Go into i386 and delete bootfix.bin (unless you want to hit any key to boot from cd)
6. If for some reason you want more than one xp version on a disc rename i386 and then take a hex editor and replace any occurance of i386 in /w2ksect.bin and /i386/SETUPLDR.BIN with whatever you are naming your folder (probably a good idea to keep it short) this works fine with the recovery console only but i have no idea what additional settings would be required for a full xp install
7. In /custom/custom.cfg add the entry for winxp recovery console
MENU LABEL XP Recovery Console
I had a feeling it might have been due to the switches -N or -D mentioned in another post so I tried them and still no luck.
Same files off a working v4 so it doesn't appear the files are the problem, so I'm tending towards mkisofs, or it's switches, or the kernel that's loading w2ksect.bin??