I still use XP and Windows 7. As a user. I don't find one much better or worse than the other, making a voluntary upgrade unlikely.

Don't expect them to learn Linux now - even if it is a better option. Security Essentials or Avast should help. It would be better to encourage them to use Google Chrome or, better, to use Firefox with NoScript, HTTPSEverywhere, though NoScript can be a bear to learn to use effectively (This, only because of recent concerns about Google's on-off relationship with the idea of having respect for users' privacy). Windows 7 or later should ensure they can quickly give new programs admin permissions if required, even if they are using an account with few privileges.