I have a el-cheapo toshiba laptop (sattelite L505) that came
with 64 bit win7. I installed 32 bit Ubuntu on it with no problems,
and the 32 bit VirtualBox. If you are installing the 32 bit ubuntu,
you want to download the i386 version and install that one. It
doesn't matter what your processor is but how the kernel is set up
(that is my understanding and experience, anyway - maybe someone can
correct me). If you install the 64bit ubuntu, then you need the other
Vbox download- though you may have the chroot issue there as
ubuntu64bit is i686. I don't know what or how that interacts with
something designed for amd64. I don't understand that stuff too well.

"uname -m" in a terminal will return your Kernel architecture, fwiw.

Regarding the installation error, it is probably just a bad burn or
download of the iso.

Also, I really think the chroot 64 stuff was for installing a 64bit
guest on a 32 bit host, but I don't know for sure.