This was what I thought until I installed ubuntu. Maybe it's the honeymoon period, but it just works. I doubt your mum is doing heavy graphics work or playing games and those are the only two things that are keeping me on win 7. The interface ubuntu is nice, it's stable and programs can be installed in one click.
I'm not saying its perfect, just that it's much, much better than I thought.
The problem with linux, for my parents, is that neither of them are really computer literate, everything they know how to do is because they've been told how to do it, not because they've picked it up themselves. There are classes in how to use Windows, their family/friends use Windows and can give advice...
If they moved to linux, then not only would I have to spend a fair bit of time getting them into it, but I'd then be the only person for them to call with issues in the future.
Yeah, right. Next time your parents accidentally break Xorg's config, you tell em to go console mode and do some heavy VIMing.
Excuse me for rudeness, but most people who think anyone's mums can handle linux are very, very wrong because I DID tried it.
The exotic technology need which required an engineer to cast magic spells last time was, ahem, wireless. I know, not Ubuntu's fault, but Mom doesn't care why it is a paperweight. She doesn't know what a driver is.
I'm not saying its perfect, just that it's much, much better than I thought.