trouble with shopping cart

Hi there i have succesfully setup and are running a version of zen so i decide to setup another 1 on another server and i have ran into this problem: you can browse the items we have but when you try to add an item to cart it throws a blank page saying done but if you click back on the browser the item is in your basket.
also the same happens when you try to login to your account here is the link
www.buynewandused.co.uk any help appreciated thanks in advance from gav

zen version Zen Cart 1.3.8a

Couple of suggestions for you.

Download and run the Debug Utility - that should help you identify the problems you are having and how to fix them.

WOW - optimize your site, it takes forever to load.

Yup - your images are 240K+ - try the Image Handler 2 mod.