Firstly, you have to check if there are multiple versions of PowerPoint installed. You can do this from Control Panel > Programs and Features > Uninstall a program. If there are more than one version, remove the oldest one and reboot the computer, then you will be able to open the files normally.
Secondly, repair the PowerPoint 2010 installation using the same instructions. I believe the software is corrupted because of another program's installation.