I've been trying to make the boot CD with BartPE included in the UBCD download. When I click on the build button, with options set to create an ISO, I get the following error:

Build ISO image
Running external program
C:\Documents and Settings\elford\Desktop\ub4\mkisofs.exe -iso-level 4 -volid "The Ultimate Boot CD for Windows" -b bootsect.bin -no-emul-boot -boot-load-size 4 -hide bootsect.bin -hide boot.catalog -o "C:\Documents and Settings\Admin\Desktop\UBCD4WinBuilder.iso" "C:\Documents and Settings\elford\Desktop\ub4\BartPE"
Error: CreateProcess("C:\Documents and Settings\elford\Desktop\ub4\mkisofs.exe -iso-level 4 -volid "The Ultimate Boot CD for Windows" -b bootsect.bin -no-emul-boot -boot-load-size 4 -hide bootsect.bin -hide boot.catalog -o "C:\Documents and Settings\Admin\Desktop\UBCD4WinBuilder.iso" "C:\Documents and Settings\elford\Desktop\ub4\BartPE"") failed
Building done...
There where 1 errors and 0 warnings

This is the only error in the build? What can I do to fix this so I can make an ISO?

You needed to actually follow the instructions found in the How To Build pages which plainly states "Do NOT extract the project into a folder that includes spaces in it's name, such as on your desktop or in My Documents"... see http://www.ubcd4win.com/howto.htm

Extract it to C:\UBCD4Win or other directory instead.