I'm using IE8 and keep getting pages come up with the little warning sign bottom left corner and usually the message error on page.
Double clicking the message gets a message box saying 'Errors on this webpage might cause it to work incorrectly' which often proves correct.
The most common message in the body seems to be "JSON is undefined"
Can anyone suggest a solution??

Have u recently had the DivX add ons installed in IE8 ?

Try disable it in IE8 (Tools-> Manage add-ons then restart IE8) and see if the JSON (JavaScript Object Notation) error would disappear.

I thought that had worked becos there were no messages for a while, but the last one that came up as a redirection from another site had the message back.
Thanks for trying to help

Well, if u visit a site that changes your settings, u should not be surprised at the outcome. Just keep an eye on the settings to see what have been altered.

PS in general, it is useful to leave a situation update whether the problem has been solved or not solved.