The easiest way would be Control Panel\All Control Panel Items\Recovery
When you click "Open System Restore" you should get some options. Choose one that's before the problems, as you understand it. Go as far back as you can. Or / and uninstall anything new you've got.
The fact that Windows won't over install itself is a precaution, made for your original OS' safety, thus not to run over what you have. It's actually a good thing, even if it may seem to be a nuisance.
There's something here I don't understand, hopefully more professional members will answer you.