Maurice
The iso you have installed on the USB Key at the moment is that the one that at some time worked on the DELL820 or have you used liveUSB since then?
The quoted menu.lst by Just18 I don't think line up any longer, you have better show us the menu list in full you are working on. 
No the iso that worked on the Dell was a Minime.iso just as a tryout. Now I have the fulll PcLinuxoskde version on the usb
Present menu.lst
timeout 10
color black/cyan yellow/cyan
gfxmenu (hd0,1)/boot/gfxmenu
default 0
title KDE ##
root (hd0,1)
kernel /KDE1/vmlinuz livecd=KDE1 initrd=initrd.gz fromusb root=/dev/rd/3 acpi=on vga=791 splash=verbose fstab=rw,auto,noatime
initrd /KDE1/initrd.gz
title KDE with Persistence
kernel (hd0,1)/KDE1/vmlinuz livecd=/KDE1 fromusb root=UUID=a2c87e7b-16a3-41c8-8b4c-7f626b3abfb5 changes_dev=UUID=a2c87e7b-16a3-41c8-8b4c-7f626b3abfb5 acpi=on fstab=rw,noauto vga=791 quiet splash=silent vmalloc=256M
initrd (hd0,1)/KDE1/initrd.gz
title KDE with Copy to RAM
kernel (hd0,1)/KDE1/vmlinuz livecd=/KDE1 fromusb copy2ram root=UUID=a2c87e7b-16a3-41c8-8b4c-7f626b3abfb5 acpi=on vga=791 quiet splash=silent vmalloc=256M
initrd (hd0,1)/KDE1/initrd.gz
title Memory Test
kernel (hd0,1)/boot/memtest-4.20