[64 bit] Dual boot, default boot menu option won't boot, others will

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http://ubuntuforums.org – I have finally got my dual boot working - I was always pure Ubuntu before but now I need Windows for work. I've even got the heat under control from my ATI chipset - better than in Windows. But... When I switch on my computer the boot menu comes up. If I select either Ubuntu or Windows they boot up fine, but if I just leave the default selected (which is Ubuntu) it starts to boot but never completes, hanging at a grey screen sometimes, a prompt in the top left corner sometimes, even the word Ubuntu and some red dots sometimes. Why does it boot OK from a selection, but not a default - isn't (Hardware)