Very strangely IE is a sod to clear out the cookies/cache. You can tell it to delete them, yet when you come back to the site they magically re-appear as before they were deleted (i.e. not recrated fresh, but copies of the old ones). Takes a few goes sometimes to get it cleared right.
I have also found, dont use the prompt at the top of the index page to login. Make sure you click through to sign in ( http://350z-uk.com/forum/ucp.php?mode=login ). I need to have a look at the one at the top of the index page as it doesnt always seem to work.