It happens to the best of us - every so often you just need to uninstall something, and it doesn't go quite right...
If this ever happens to you with Modeler 2012, and you find that you aren't able to reinstall after an uninstall (you might get a message that the product is still installed), it's time for a quick foray into the the registry. As always, be careful in there.
To start, search for registry entries starting with ‘70D63E85’ under ‘HKEY_CLASSES_ROOT\Installer\Products’. You'll probably find these:
Delete them, and try the install again, and have fun! (personally, I'd recommend trying out 2013 while you are at it - there's a lot of fancy new stuff in there!)