I'm running Office 2007 on Windows 7. When I ran the Microsoft Office 2010 beta installation, it asked if I wanted to upgrade the existing 2007 installation or install along side 2007. I later uninstalled 2010 and rebooted. When I re-ran the Office 2010 setup program, it no longer asked if I wanted to upgrade or install side by side. It just did a side by side install by default. How can I get it to re-prompt me, as I'd now like to upgrade my 2007 install.
Presumably there were some elements of 2010 left behind when you uninstalled which knew that you were, in a way, upgrading 2010. Try to uninstall again using Revouninstaller. This will (more) ensure that everything is gone. Then reinstall.