Internal Server Error at checkout

Hi
I get an internal server error at checkout can someone tell me what it could possibly be from
Thanks
https://www.imperialgiftsnwares.com.au

This is what I have in my error log
[Sun Apr 27 17:46:57 2008] [error] [client 124.184.221.226] File does not exist: /home/vol2/imperial/public_html/404.shtml, referer: http://imperialgiftsnwares.com.au/index.php?main_page=index&cPath=7
[Sun Apr 27 17:46:57 2008] [error] [client 124.184.221.226] File does not exist: /home/vol2/imperial/public_html/includes/templates/custom/images/homepage15.jpg, referer: http://imperialgiftsnwares.com.au/index.php?main_page=index&cPath=7
[Sun Apr 27 17:39:18 2008] [error] [client 124.184.221.226] File does not exist: /home/vol2/imperial/public_html/500.shtml, referer: http://imperialgiftsnwares.com.au/index.php?main_page=checkout_confirmation
[Sun Apr 27 17:35:39 2008] [error] [client 124.184.221.226] File does not exist: /home/vol2/imperial/public_html/500.shtml, referer: http://www.imperialgiftsnwares.com.au/
[Sun Apr 27 17:22:31 2008] [error] [client 124.184.221.226] File does not exist: /home/vol2/imperial/public_html/404.shtml
[Sun Apr 27 17:22:30 2008] [error] [client 124.184.221.226] File does not exist: /home/vol2/imperial/public_html/favicon.ico
[Sun Apr 27 17:22:29 2008] [error] [client 124.184.221.226] File does not exist: /home/vol2/imperial/public_html/404.shtml, referer: http://www.imperialgiftsnwares.com.au/
[Sun Apr 27 17:22:29 2008] [error] [client 124.184.221.226] File does not exist: /home/vol2/imperial/public_html/includes/templates/custom/images/homepage15.jpg, referer: http://www.imperialgiftsnwares.com.au/
[Sun Apr 27 17:02:24 2008] [error] [client 124.184.221.226] File does not exist: /home/vol2/imperial/public_html/404.shtml
[Sun Apr 27 17:02:24 2008] [error] [client 124.184.221.226] File does not exist: /home/vol2/imperial/public_html/favicon.ico