Live boot not working

I get the first menu screen and starts to boot into live from usb (Ubuntu loading screen) then I get the following....

BusyBox V1.18.5 (Ubuntu 1:1.18.5 - 1ubuntu4.1) built-in shell (ash)

Enter 'help' for list of built-in commands.

(initramfs) unable to find a medium containing live file system.

If I press Enter a initramfs prompt appears. That's as far as I have got.

MD5Sum is OK.

Trying this on a HP xw6200 dual xeon 3.2ghz. I'd like to dual boot with XP on seperate hdd..... If I can get it working?

Any help much appreciated. Thanks.