It took me a whole lot of time to finish the upgrading process.
I followed the instructions of upgrading from 6.10 to 7.04 posted
on ubuntu's website. I then pressed "replace" to replace some
configuration/spec files as suggested during the upgrade process.
It said the system was upgraded successfully, i.e., nothing
wrong happened.

But now I cannot boot the system any more. I failed to boot from older
versions, maybe they have been removed. So what can I do now?
I do not want to reinstall since the first time I installed 6.10, it gave me
a lot of hard times.

Thanks.

exactly what error message do you recieve when you try to boot?

I figured out the problem is due to external video card I am using.
Could boot using the onboard video card now. Looks like some
configuration files were overwritten, working on it. Will get back
to this thread if I can get it solved.

Thanks buddy.

As I said, the problem was due to onboard video card.
What I did was: add this to the end of /etc/modprobe.d/blacklist
blacklist agpgart
blacklist intel_agp

I also set up to use ext video card in bios, changed /etc/X11/xorg.conf
accordingly. Then everything back to work.

great! glad you got it working!