Grub and live cd

[John Levin]

Hi,

I'm trying to test the live cd on a computer with a fully-functioning 
ubuntu / win98 installation. But instead of booting from the cd, it 
goes into Grub, and a plain command line.
CD booting normally works on this machinbe - done it with many 
different live cds (knoppix, slax etc) - but I seem to recall having 
this difficulty once before, and it may have been something to do with 
Grub.
Suggestions?