If you want to customize UBCD, you better use UBCD50 (beta stage for the moment), because it is better structured.
The /boot/isolinux/ (for CD) and /boot/syslinux/ (for USB) folder is needed for booting.
/antivirus/ contains definition files for avira antivirus
/pmagic/ contains Parted Magic
/ubcd/menus/isolinux/ contains the config files for the menu entries.
/ubcd/boot/ contains utilities that use a kernel and initrd (so linux based programs).
/ubcd/custom/ is the place for custom images added by users
/ubcd/images/ contains floppy images that are emulated with memdisk.
/ubcd/dosapps/ contains programs that are used together with the freedos image (/ubcd/imges/fdubcd.igz): http://www.ultimatebootcd.com/forums/viewtopic.php?t=1422
/ubcd/tools/ contains tools to rebuild the iso if you modify it and a script to put UBCD on a USB thumb drive.
for information about how the isolinux config files work.
Look in the menu config files at /ubcd/menus/isolinux/ to see which files or directories you can remove if you don't want a certain program.
Use a virtual machine to test your newly build iso (e.g. VirtualBox).
I want to use UBCD just for Switching between my LIVE OS ( Linux , DOS )
You just can use grub for that. You don't need UBCD at all.