Sharepoint Question

Hi
This is abdul Javed Khan.

I have one question that when i am accessing sharepoint site by specifying url of the site, in some system it is asking crediantial, but in some system when i am opening sharepoint site it is taking our exchange account(username & password) as default crediantial.
I want to restrict this that is whenever i access the site from any of the system(machine), i want that it will ask the username & password from us.

So how can i do that, i am using Microsoft office sharepoint server 2007.

Please tell me the solution
thanks in advanced

Regards,
abdul Javed Khan