Would guess since you upgraded from Vista, that you have the Upgrade Retail Win-7 disks. When upgrading, you have to upgrade from the 32 bit version to the Win-7 32 bit version, or if Vista was 64 bit, from the 64 bit version to the Win-7 64 bit version. Can't upgrade from 32 to 64 bit - not possible.
So you would need the Vista 64 bit program to upgrade to Win-7 64 bit version. To install Win-7 64 bit version cleanly, you would need the full retail or OEM version to do that.
You need to perform a clean install not an upgrade if you want 64 bit windows 7.
You can use your 64 bit windows 7 upgrade disc but you have to perform a clean install !
Just make sure your computer is 64 bit capable !
To install Windows 7 using the Custom option and formatting the hard disk
To format your hard disk during Windows 7 installation, you'll need to start, or boot, your computer using the Windows 7 installation disc.
Turn on your computer so that Windows starts normally, insert the Windows 7 installation disc or USB flash drive, and then shut down your computer.
Restart your computer.
Press any key when prompted, and then follow the instructions that appear.
On the Install Windows page, enter your language and other preferences, and then click Next.
If the Install Windows page doesn't appear, and you're not asked to press any key, you might need to change some system settings. To learn how to do this, see Start your computer from a Windows 7 installation disc or USB flash drive.
On the Please read the license terms page, if you accept the license terms, click I accept the license terms, and then click Next.
On the Which type of installation do you want? page, click Custom.
On the Where do you want to install Windows? page, click Drive options (advanced).
Click the partition that you want to change, click the formatting option you want to perform, and then follow the instructions.
When you've finished formatting, click Next.
Follow the instructions to finish installing Windows 7, which include naming your computer and setting up an initial user account.