Thanks for replying, took the pc today to my local IT shop for them to look at - they diagnosed spyware/malware that conflicted and it couldn't be removed. It would need a reformat from scratch which would cost £55 - thus will have to rely on AVG free 2012 which has installed, though AVG free 2013 also failed. Its an oldish machine anyway and not worth the £55
AVG have very kindly refunded me the cash which is good of them.
A lesson to learn is to try the free trial period first to see if it works !