If you let Arc install Python for 10.0, then used the uninstaller for Data Interop, then uninstaller for ArcMap - everything should have been left in a good state.
Its possible you're still seeing stuff in the Python 2.6 directories because you installed 3rd party modules after the fact (Arc wont clean those up).
If you did your own Python install for 10.0, Arc uninstaller wont remove it. Because you dont have the option to uninstall, again it sounds like you let Arc install it and the core pieces of it went away when you removed 10.0
If I were to tell you to go and delete directories without understanding your system completely it would be irresponsible of me. I'd suggest contacting Support and they you can help you "clean up" and most likely do a "re-install".
Thinking more about this though - within the Python window, can you do an "import arcpy"? If you can do that, it might not be as bad as it seems. It could be something specifically in your script (like a dependency on a 3rd party module) which you haven't yet installed for your Python 2.7 deployment. The error you posted might be cryptically saying that.