I can't seem to get those Ubuntu USB install to work

Yesterday I tried two Ubuntu 8.04 installs to USB from Pendrivelinux.com: a full install and a (for lack of a better term) lesser install. Neither worked.

• The full install booted and asked me to log in but then went to a blank screen - the desktop never loaded. I could Ctrl-Alt-Del and get the shutdown options, but that was all.
• The lesser install (it only requires a 1GB usb drive) would not even boot.

Both are install from the live CD which works fine. Both I tried both twice to no avail. Suggestions welcome.