I tried to open "about config" in windows 10 and I get a pop-up saying 'you'll need a new app to open this'.
Is this normal? If so how do I do it co's nothing happens when I click on the ok box but to be fair it is greyed out.
'about:config' isn't a Windows command (or not as far as I know, anyway!). It's a browser command. So what are you trying to use it in? (Edge?, IE11? Firefox? Chrome? Something else?) And have you remembered the colon (which you've omitted in your post)?
The nearest command that I know of for Windows itself is 'msconfig', rather than 'about config'.
Ok, I'm doing this for a pal and msconfig works fine. As soon as it opened he said 'that's it, that's what I want'.
So brownie points for me....or it would've been if he hadn't seen it was you who solved it!!
Many thanks.