Ian,
unfortunately DC is not tested or validated for 64bit OS, ergo it is not officially supported. Some users have had success in running on Windows 7 - 64, but it seems to depend upon OEM version and their hardware configuration. There is no clear pattern for success or failure.
Some resolutions have included turning off UAC (user account control), intalling and ruuning As Administrator, and as you've already tried Turning of virus checking.