I am currently trying to render a file I created in 7 on 6.2.1 because after about 40 failed attempts on 7, I just gave up. :)  I usually run Vista at home, and I never saw the lockup problem with 6.2.1  Maybe 7 just doesn't like something in his system...  I usually have to render overnight since I sometimes use a high DPI setting because I like to get poster prints of some of my better renders.