What has probably happened is that AVG has detected a change to the ntoskrnl. This is most likely following a windows update being downloaded which has then made the changes. If you are not experiencing any other problems then I wouldn't worry. You could accept the change in AVG and then you wont get the notification in future.
Beware of any sites offering fixes as generally they do a partial scan - tell you you have problems,which you dont, - and then ask for a payment to download the product.
Run an anti spyware programme just to make sure that there are no nasties lurking in the system.