I have not installed x64, but if it installs like 32bit, let the install run
untill you get to the part where it asks if you want to do an upgrade or a
custom install. Just under that box under the right corner it says advanced.
You should see format. Click on that say yes and continue install.
If you are currently running Windows XP x86 on the computer, you must boot
from the DVD in order to install Vista x64. You can not install a 64-bit
operating system from within a 32-bit operating system.