Internal Server Error error 500

[makismakana]

Hi,i'm new user of ZC ,but i like it and tried to install it to my home server witch i have for test purposes. It includes apache 2.2.2 ,php 5 and mysql 4.0.18. Thats my home server. ZC works fine there.
After testing and configuring and getting fammiliar with ZC i tried to install it from the begining to my webserver, and then uploading the additional files i needed. So i uploaded it installed it and everything was fine. My webserver is in linux with mysql 4.0.25-standard , with php 4.1.2 (i know it's not supported by ZC but i tried installing it by changing the 4.0.2 permision to false in the zc_install\includes\modules\pages\inspect\header_php.php (line 395) folder,and finaly made it)The ZC version is the latest 1.3.0.1

As i said it worked fine, the admin section and the main shop. But then i realised that when i try to buy something as a user that is not added to the shopping cart and gives an 500 error.

Then i tried to install a older version (zen-cart-1-2-7-d_full-release_bugsecurityfix) and it also gave an error in the first installation page but it continued.I didnt change anything. I did installed it and tried the shopping cart module, but the same sh*t. Internal Server Error.

So i looked in my error log and it shows that in both installations:
[Sun Jun 4 16:14:13 2006] [error] [client MY-IP] File does not exist: /home/*****/public_html/500.shtml
[Sun Jun 4 16:14:13 2006] [error] [client MY-IP] Premature end of script headers: /home/*****/public_html/eshop/index.php

the link for ZC 1.3.0.1 is http://www.*****.gr/eshop/
and the one for 1.2.7.d is http://www.*****.gr/e-shop/

The case is database , file , and the case folder path is correct and chmd 777.
I have made the installation several times and nothing helped.

I have tried everything from the older posts in the past 2 days but couldnt find a sollution. So if anyone can help me i will appreciate it.

**edit: i forgot to mention that if the shoping cart is empty it shows.when there is a new product added it returns a 500 error