|Having some experience with this (I've set up several machines for the elderly), I think I can provide some additional points (irregardless of OS). Some (or maybe all) of this, I'm sure, you've already thought of:|
1. Set up only what they'll need on the desktop. Provide shortcuts in order of their usage (most used first, obviously). As you feel more comfortable with their progress, add more as needed.
2. Use large fonts/icons and a simple (single color) background. No need to explain this one.
3. Install anti-virus/malware programs (if necessary---Linux/Mac IS more secure), but try to have them update in the background, sight-unseen.
4. Provide links in whatever browser they may need (retirement, banking, senior-friendly social sites, etc) and make sure they understand how to find them. If necessary, provide direct links on the desktop.
5. Install some simple things for fun (card games, checkers, etc.) that you think they'd like. If possible, set the difficulty on the games to be challenging to them, but not to the point of being impossible to beat. Again, make the display as large as possible and provide easy to find links (perhaps an entire "Games" folder on the desktop).
6. Setup a web-based email client with decent built in security. Yahoo! Mail used to be ideal for this with the automatic deletion of trash/spam, but that feature apparently no longer exists. GMail would also be an alternative, but it's interface is somewhat less "user-friendly". Make sure they know the basic rules of "unknown sender = unopened mail".
"Channeling the spirit of jboy..."