Extract the initrd and kernel using p7zip-full (7z x Wubi-netboot-7.04.exe), then place those files into the /boot folder of your Slax Linux USB install, then add to /boot/grub/menu.lst in the Slax USB install:
This sounds good... but what if I don't have Windows installed on the computer? I have a computer with an empty HDD, and Slax Linux on a bootable USB stick. Do you know how to netboot from this setup?
Substitute the names of the kernel and initrd for the real ones, and remeber to set your timeout to greater than 0, then boot the Slax install, select Ubuntu when the grub menu shows up, and it should start up the netboot.