After install of 12.04, I get message: error: unknown filesystem. Dual booting.
 

I'm dual booting with Win 7. Have had 32 bit version working fine, but wanted to use 64 bit. I deleted / directory, but left /home alone. On reboot I get "error: unknown filesystem" message.

You mean after deleting / you get unknown filesystem?
Did you install 64-bit after deleting, or did you delete & expect it to boot with no grub as grub is usually part of /.

Hi there and welcome to LQ.
Well as said in the previous post when you delete the / of linux evrthing is deleted and so no boot is possible.
Use a live cd/dvd and back up your /home (if you feel that your data is important. Then you would have to reinstall linux.
Hope this helps

The Master Boot Record (MBR) is on the first sector of /dev/hda. It was not on the / partition where I had Ubuntu installed. I did not delete that sector. I did delete the / partition which was on /dev/sda7 where the OS was priviously installed. I left /home alone. It was on /dev/home on /dev/sda5.

Thanks for the welcome, honeybadger. I did backup /home before I began the install process. When I check gparted on the machine I am now on, which is running the 64 bit version of Ubuntu 12.04, I get the following location for the boot sector:

/dev/sda1 is the only partition with a "boot flag"

The MBR doesn't contain grub it only points to it, Your grub was in your / partition.
If you already have your installation media then you can go ahead & reinstall os.
How did you backup home directory? Just the files or an image of the partition?

Also linux doesn't care about the boot flag. It redirects to whatever partition grub was installed to.