Go to microsoft site and get free Windows Installer Cleanup tool. Be very, very careful using this tool to remove autocad if you still can't uninstall autocad via control panel. Once this tool is used, there is no going back. Also, after using this tool to uninstall autocad, make sure there isn't anything left in the autodesk/2011 folder. Reboot, turn off antivirus and re-install.
I had to use this tool once, so don't be too concerned, BUT, try to remove first via control panel. Afterward and after rebooting, check control panel to make sure 2011 isn't listed.
I forgot to mention that you may have to edit the registry before rebooting to remove ALL instances of autocad and/or 2011. Before doing this, however, it's always a good idea to export the registry as a precaution.
"Windows Installer Cleanup" tool was retired from Microsoft site. But from here http://majorgeeks.com/ it possible to download