Mike,
Without knowing exactly how Ubuntu deals with a new installation on a disk that
already has partitions (and not being an Ubuntu user) I cannot tell you that I
have a cookbook solution. But what I can tell you is that any sane distro will
ask you were to install the root directory "/" and certainly whether to do
any formatting of the existing partition. I'd rename the /home to /home_OLD
and go with a fresh install step-by-step minus any formatting.

Booting and having a proper boot-loader is the crucial part. It will likely
place grub where it should be in a partition that will be mounted as /boot
under the new system. In essence, even if you wiped /bin /usr and all of that
clean, the system would be new from scratch anyway and with /home_OLD untouched.

If you really paranoid, mount a USB drive with any filesystem on it (it can
be an NTFC) and do this:

'rsync -av /home/ /mnt/where_USB_is_mounted/home_TRANSFER/'

You can always recover from there if anything goes wrong.