Tried Esc - that just restarts the computer, which just restarts memtest.
There is no USB or CD in the computer - it is booting off the hard drive.
I have pressed the power button - same result as Esc.
Memtest is installed with the Ubuntu package (My computer was running version 11.04), hence why it does not need a CD to run. I have found no BIOS option to disable it.
More poking around seems to indicate there is something wrong with the grub menu, as its only options are to boot memtest.
Malmental - my username is fine, considering that none of you have found a solution to my problem either.