OK what about checking Internet Explorer defaults? Make sure you check associations as well - press the Windows key and type 'default' and open the control panel link. Then make sure IE is not selected on any of them as this could prevent Chrome being chosen.
Also when you reinstall Chrome, remove it fully - delete any leftover files (usually left in C:/Program Files).
EDIT: Is it just Chrome that refuses to be default? Have you got any other browsers you can test with?