As an additional note I looked at the bitpay.php file referred to by previous posters and line 146 is empty.
EDIT: I found the log and the error I am getting is different.
[03-Aug-2013 14:19:01] PHP Fatal error: Call to undefined function zen_remove_order() in C:\inetpub\public_html\zencart.mydomain.net\includes\modules\payment\bitpay.php on line 137
Line 137 reads:
zen_remove_order($insert_id, $restock = true);