Hello and welcome to JustAnswer, my name is ***** ***** will be helping you today.
From what you described, it sounds like you may have downloaded the wrong bit version, or maybe it corrupted while installation, because it was wrong version, and now you get an error message.
Good thing is, that you have a serial number, meaning, you are good to go, once you have the correct version to install.
Why it took 2 days to download is unknown. While this program has a large install file, maybe 1-3 GB. If you have highspeed broadand, it should only take an hour at the most....even on slow broadband.
I think the best way to approach this, is for me to connect to your computer, so I can see, and make changes, while you watch. This is honestly the best and most accurate.