It all about her needs and expectations.
When we finally convinced our grandma (now 93) to get a computer for email almost 10 years ago, we initially gave her an older mac thinking it would be easy for her.
However, she has basically zero computer experience so understanding basic UI interfaces is out the window.
However, over time she starting having problems because she couldn't understand how to manage overlapping windows.
She would accidentally minimize a window, couldn't figure out where it went and would open up a new one.
Repeat this 30+ times on a computer with 512MB of ram and you life becomes utterly painful... It would take like 3-5 minutes to open up a new window, because there was like 2GB of swap in use.
I tried to explain to her what she had been doing many times, but the concept of overlapping windows was just beyond her comprehension.
I ended up installing linux on it and hard coding x to run firefox at startup, bypassing a window manager entirely.
When she quits firefox, it kicks her out to the login screen.
That's it. No menu bars,no tabs,no background processes - just one big window.
In this case, KISS isn't just a goal - it's a requirement.
If it's humanly possible to switch to a different program, it will happen with her and she can't figure out what happened.
True, It drives any visitors crazy (until they read the sticky note that says use the guest account that brings up a conventional desktop) but it's has caused the least grief.
It would be very difficult to do something similarly with windows/mac.
I had looked around and there was a number of KIOSK type programs, but they wanted $100+ for the trouble.
All she cares about is her email. I setup the homepage as her email and it auto-logins in with the remembered password.
She doesn't even use Google for searches.
Don't ask me why The Cousins switched her to Google mail (which is a horrible choice because the UI changes every few months and I have to re-explain everything over again). I've tried to switch her back to the simpler thing I had but she doesn't want to budge...
If I were to start over again, I'd probably get her a chromebox (needs a big desktop screen) or maybe an ipad (if she could get comfortable with the whole swiping thing).
If you want a XP like experience, LXDE is really hard to beat (not to mention super lightweight).
Icewm is another option, if you want something that is even simpler with a similar layout.