First of all, your activation code for 32 bit is equally valid for 64 bit.
If you do not know it, run a free app called "magical jellybean keyfinder"
You need to do a clean install.
It sounds like the copy you downloaded might not have been good.
Download the proper 64 bit version a rgd1101 linked.
If you have not yet converted to a ssd, now would be a good time to do so.
Do your windows install to a new ssd, leaving your old drive disconnected.
Boot from the dvd and do a clean install.
You will need to identify the windows version(home/pro/...)
At activation time, enter your old code and you are good to go.
You will need to reinstall all apps since you have a new registry.
Data files can still be read.
If you have settings you want to move, first export them to a file using windows easy transfer.
After 64 bit is installed, you can import the settings.
You will also get a checklist of apps that need to be dealt with.