I gave Ubuntu 7.10 AMD64 a fair try on my workstation mainly as a curiosity, and while it was somewhat usable (and Compiz Fusion was very nice and relatively bug-free) there were too many issues for ...
It's probably Windows setting your UEFI (aka what used to be called BIOS) to boot from the Windows bootloader partition instead of Grub. It's probably still there if you set the right partition in the ...