Comments problems

Hi again,
I have gotten the whole site set up properly but have one final problem that I don’t know how to fix: when a user leaves a comment, the page just turns blank.
I have put a completely fresh copy of all files (i.e. the wordpress defaults) back up for all except the wp-layout.css which is now set up for the human condition template on alex king’s site.
if you think it will help, go try leaving a comment at http://www.unit77.co.uk and see if you can work it out because i cant 🙁
Thanks again, Richard Ellis