I read recently someone's account of meeting someone who had no idea how to use a computer (for whatever reason, he was a middle-aged westerner who was always afraid to try to use one).
The usability person who wrote the article noticed he had no idea how anything worked. Not how to use a mouse (I've seen this myself with seniors learning Windows; the concept of moving something below with your hand while something else moves along-with is not easy to get a normal brain around). Not what links meant, how to use a browser, where to get help (and even if he did get help, he didn't know what the help meant).
Frankly, when building a web page, you have to make some minimal assumptions, no matter who the users are:
-they have an internet connection. Yes, if you don't have a computer or access to the internet, then your web page is inaccessible. There is no reasonable solution to this, so don't waste your time.
-they know how to use a computer. Now here, your users don't. That's fine: they will have to learn to use the basic hardware before anything else makes any sense to them anyway. You wouldn't "dumb-down" all the books in the library because someone might enter who was just learning the alphabet, nor would you demand all the books therein were super large-print because some users have bad eyesight. Your users are diverse and when you know your site is targeted to new, ESL, or fragile users, then you do take them into account and build things with them specifically in mind... but your site is also teaching them computer and web site conventions, remember. They will have to learn what a hyperlink is whether it says click here or <a>look at my vacation pictures!</a>. So let your site be a great place for them to learn those conventions.
Are there any accessibility guidelines available about writing websites for absolute beginners? All the ones I've encountered seem to assume that the user has at least a nominal understanding of how to use the internet.
They have to make that assumption just as they have to make the other assumptions (the user knows what a mouse is, the user speaks English, the user is literate), though the best guides will have tips for when your users don't fit those assumptions: can't use a mouse, uses a screen reader, doesn't have popular software or popular 3rd-party plugins.
I read Jakob Nielsen's [alertbox, older articles from [url=http://uxmovement.com/]UXMovement, and books like Steve Krug's [url=http://www.bookdepository.com/Dont-Make-Me-Think-Steve-Krug/9780321344755]Don't Make Me Think!](http://www.useit.com/alertbox/).
None of those assume the users are absolute beginners, because the majority of users usually aren't for most sites. Kinda like there aren't how-to-use-roads for people who don't know how to work cars. They have to assume you know how to shift, steer, etc.
When people are learning something for the first time, esp if they are not children, it's the hardest part of using anything for them. This is why we use conventions: they are learned by people and then allow people to auto-pilot those things and concentrate on the subjects or tasks at hand that they came to do.
So I'm saying: give them good conventions to learn. They will encounter bad pages as well, and be confused probably, but they'll learn those as well.