My consulting practice, Geezer Computer Education and Consulting has worked with seniors for over 10 years. I am the Senior Geezer who came from the world of Information Systems.
(1) Remove all bloatware
(2) Install Norton Security Suite 2009 that offers a lot of protection very quietly (senior men watch a lot of porno)
(3) Low resolution on a large monitor - most like it at 800 x 600, but on very large monitors that can be higher
(4) Install Cloudmark or other strong spam tools
(5) Insttall Netcraft or other strong phishing tools
(6) Set them up with gotomypc and monitor their updates for them. Many seniors pay no attention to the system tray.
(7) Set them up with Roboform Pro and sync their passwords to a USB drive
(8) Automatically Sync favorites and pictures to USB drive
(9) Spend many days showing and explaining to them what a Window is, what a dialog box is, what a pull down is. There are many studies showing that Seniors have reduced 3D vision and have trouble distinguishing between the dialog box and the background window.
(10) Get rid of any programs not currently used. They can always be installed later.
(11) Teach them, using gotomypc, the difference between downloading and installing
(12) Teach them the most common terms they will need to explain the problem (monitor, computer, mouse, keyboard, router, modem, etc.) Do not assume understanding until they can explain it back to you.
I cannot emphasize enough the value of good remote control. Also, though Ubunto may be fine, they will not be able to get assistance as readily as if they have Windows. My seniors have actually loved Vista, though they were initially very skeptical about moving from XP.
Good luck, the seniors you serve well will definitely appreciate you.