internal server error 500 while installing

[Prissy]

Thanks i talked to my host and he fixed the problem. i finally installed but now i have another problem, when i try to log in through this url http://www.MYSiTE.com/shop/admin/index.php

i receive this error message:
Fatal error: Cannot instantiate non-existent class: messagestack in /home/xxx/public_html/shop/includes/autoload_func.php on line 79

also when it takes me to the index page where it says 'why not log yourself in?', i click and a pop up says: "YOURSiTE has sent an incorrect or unexpected message. Error Code: -12263"

[DrByte]

You are most likely missing files on your server due to failed FTP upload.
Sometimes files get missed or get left incomplete or with 0 bytes size when an FTP connection gets interrupted. Not all FTP programs are as reliable as others, and thus not all of them will tell you which files failed to upload properly.
FileZilla is a good free reliable tool you might consider using when you check all your files and re-upload the missing or incomplete ones.

[Prissy]

i re-uploaded everything, now when i try to log in through admin link
http://wwwMYSiTE.com/shop/admin/index.php

i get the same pop up with error code 12263 =/

[DrByte]

Zen Cart doesn't use popups in any of its login activities.

I suspect your hosting configuration is the problem behind those error messages.

Who is your host?
What is your site URL?
Is your site attempting to use SSL or not?