My 64bits desktop (intel core i7) was able to boot with Windows XP 64 installed, but it's not more possible after installing Ubuntu amd64: the computer says the HD is not bootable. The only way of booting the computer now, is using the Live CD and choosing boot from the hard disk. Besides, if I put this HD in a 32bits computer, grub boots normally. How can I enable boot directly from the HD using the 64bits desktop?
Sounds like you need to reinstall grub (or at least have it embedded on the MBR). Grub does usually work fine in a 64bit environment (at least it works here).
There are a lot of guides out there for doing this but most concern Grub v1. Ubuntu has been on Grub v2 (confusingly the actual version number is 1.9x) for a while and the repair instructions are slightly different.
If I were you, I'd follow this guide and see where you get.
Following my last comment, I googled for your motherboard and "boot flag", and in fact the Intel DX58SO doesn't boot, if there is no primary partition with the boot flag set.
You can check this with:
fdisk -l /dev/sdX
If no partition has an asterix (*) in the 'boot.' column, then that's the problem.
To fix this, you can use GParted from the live CD:
- Right click any primary partition
- Select 'Manage Flags'
- Tick the 'boot' flag
- Click on 'Close'