10.7.6 upgrade freezing

I have tried downloading and installing the latest OS version 10.6.7. It downloads fine and commences installing and writing the files but "stops" when almost completed and remains there (I left it for 4 hours today hoping it might finish). The same happened when I upgraded to 10.6.6 but somehow it completed writing - I can't recall what I did if anything!
Has anyone else had this problem? or, better still, does anyone know hoe to fix it?