I do think that some kind of basic guide should be in the manual.
Just a list of things to watch out for.
If you get a popup saying you've won an ipod, and you didn't enter any contest to win one, then you haven't. Stop clicking random stuff.
IF you get an email from you bank saying your account has been hacked and you need to enter your information, yet it's full of spelling mistakes and the link takes you to some crazy URL, even tho they're using the bank's logo, it's not real, close it, delete it and walk away, phone your bank if you're concerned.
Anything offering to 'clean/speed up your system for free' is actually offering to clog up and slow down your system, then sell you the fix for lots of money.
Keeping your passwords/ network IDs in a text file on the desktop isn't the most secure option. By all means print them out, just don't stick them on the wall in full view of your webcam

(Against old advice, but it's been shown that the thieves who hack your computer are different types to the ones who'll break in and steal said computer.)
Oh and for fark's sake, LET your techy son/sister/friend SHOW you how to do things, don't expect them to just give you a step by step guide on how to do the one thing you want to know, if you let us show you how to use the damn browser in full, you wont need us to give you a step by step guide on how to view the SECOND website you hear about that you might want to see. I swear, my mother does this. Flatly doesn't want to learn anything, but wants me to show her how to get to a site from the starting point of the PC being on. Flatly refuses to know about typing things into the url bar, or google, or whatever.