My Ubuntu boot disk won't

I recently installed Ubuntu as a dual boot system with Windows but the wireless keyboard wouldn't work at boot so it always booted Ubuntu. Got fed up with it and uninstalled it. Played with the OS on another computer for a while to get used to it and decided to re-install it. The start up sequence ignores the boot disk in the CD drive and just boots into Windows. Thought there might be something wrong with the disk so burned a fresh copy onto a new disk but still no luck. I know the boot sequence is correct as it will boot my Windows disk. I have tried the 'press f12' route and all it gives me is a 'no possible BIOS found' error message. I am using the same the same process as gave me a successful install last time so I don't know what is wrong.