Shopping Cart Problem - HELP!!

Hi,

I have installed Marketpress plugin in my site. Everything seems to be OK but for one problem that I am encountering right now.

As when I add product(s) in my Cart, it will appear on my shopping cart widget that the product(s) is successfully added.

However, as when my friend logged into my site for the first time, (using another computer) the shopping cart appear to show the product(s) that I have added.

I am wondering why is this happening? How come the shopping cart showing the product(s) that I have added in the different computer? The same problem also happen when the third friend visit my site using another computer with different IP address too.

Kindly help me to resolve this problem.

Thank you so much in advance