Russ Walter’s Secret Guide to Computers is an excellent free tutorial site on all things personal computer-ish. It’s comprehensive, simple to read and just perfect for your granny.
The only caveat I would give is that it has the feel of something that was originally written quite a while ago. Why else would there be such a lot of space devoted to the ancient and all but defunct dot matrix printer? However it has obviously been lovingly updated continuously over time and is now on the 28th edition.
Apart from that small thing, this is a stonking offering. Russ even provides – oh my gosh – free telephone support 24 hours a day.