Unable to upgrade?

I have iMovie 09 version 8.9.6. and none of the downloads willl let me install. The errors say I need version 9 to install. Besides that, I can't locate the serial number in order to even register the version I have. It came pre-installed on a computer I purchased in Oct 2010 and I never tried to use the program until now. I might even have registered it when I first got the machine, but I don't remember.

What are my options now?

Thanks in advance for any advice you can offer.