"invalid partition table" trying to install Win7 from USB

I have Win 7 on a USB stick. I used this same USB to install Win 7 on my laptop earlier today, install went smooth as silk.

Then put the same USB in my desktop (Dell Dimension e521), change the boot sequence to boot from the USB first, and i get black screen with error message "invalid partition table".

I then disabled all the hard drives and cd rom, and plug in the USB and boot, and i don't get that error, just a blank screen with blinking cursor top left. no movement for an hour now.

Any thoughts guys?

Windows 7 should be on this drive ok, if it installs properly on another computer right? Or might my desktop need the USB to be formatted differently?

Stuck,
Mike