It wasn't any particular AVI really. My problem was fixed by turning down options in nvidia control panel.
I have noticed that if I open a new AVI just after one has stopped playing Media player crashes and I have to close it. In older builds, not seen this again thankfully, some AVI files would stop playing half way through and I was then told that it cannot find the file
HD files lagging and being annoying has been fixed by removing K--Lite.