The trick for me was to first uninstall previous OB attempts and reboot.
Then make sure the rytm has the appropriate firmware installed.
Make sure the rytm is connected to usb and turn it ON. If windows prompts you to install drivers when you first turn on the rytm, just say no.
Now you can install OB.
This worked for me.
I have to add that I also resisted updating to win10, for similar reasons as you mentioned. But I found that OB was a bit unstable on win7 - I could live with this but, even so, it was a bit annoying. I ended up installing a clean win10 and spent the time getting all my bits to work with it. It was not painless, but it was worth the pain.