Returning a "is currently unable to handle this request. 500 Error" when client posts
As Lindsey pointed out, I'd also highly recommend taking some time to update to the latest Craft release. Craft 1.3 is over 3 years old now.

If you're getting a 500 Internal Server Error, then it's either being generated by Craft or by your web server.

If it's generated by Craft, it will be Craft styled and the underlying error message will be in your craft/storage/runtime/logs folder.

If it's generated by your web server, the underlying error message should be in your web server's error logs.