My mum barely computer literate. She can click the 'start' menu and find "Word" or "The Internet" or "Email" (as the programs are named) because she's been taught that.
I got tired of having to go around there and remove all the crapware from her Windows machine (and she wasn't even administrator on it). It turns out that being barely computer literate means she never bothers to remember the basic don't download and run every random piece of crap from the Internet talks we have. Typical "I just want to do this thing I want to do, and I don't care what you told me" mentality of a lot of naive computer users.
I set her up a Linux desktop machine with Xubuntu. It's connected to my VPN so I can manage it remotely. Being Linux there's a practically zero malware. Not being Windows means she can't download and run a whole bunch of crapware that gets peddled by every piece of shit website she visits. Being foreign means she is less reluctant to pick up the phone and ask for help rather than going to Google and then downloading a bunch of crapware. She's yet to find something that she needs to do that she can't achieve on Linux (except install random crapware) with a little help (usually installing the occasional piece of software for her).
Conceded, the situation has improved with Windows 7 - non administrator users are really far less likely to be able to install crapware - but it's not perfect.
Now, your average tablet isn't locked down at all, and usually can't easily be. That means that your computer illiterate user will be able to go to the app store and install whatever piece of shit apps look like they might fill some need (or allow frivolous time wasting). Being computer illiterate, they never check the required permissions for apps. Even if they could be trained to check, would they really understand what they were seeing and ask the right questions? (why should Angry Birds need access to my phone book, SMS messages, email, local storage, network, calendar, etc). Tablets are great. The granular security provided by Android and Win8 (I can't speak for iCrap because I haven't used it for ages, and back then it wasn't granular or listed) is very good, but in the hands of someone who doesn't think about security it may as well just allow everything always because most users will just click the "get the fuck out of my way and install the fucking app already" button, regardless of what they see.