Maybe its the DVD ISO itself is not compatible with your systems boot sequence and/or DVD drive. One way of narrowing it down would be to try a burn of Knoppxi 3.9 & see if the system will boot off of it. If you do not want to download another 700MB you could try to boot off of Damn Small Linux http://damnsmalllinux.org/ which is 50MB ISO file. If Knoppix 3.9 or DSL fails to boot then you might look into a boot manager called smart boot manager. You can google smart boot manager for more info & you can find other folks who have used it by searching the forums for sbm.
I hope this helps.
James