Sometimes you need to log in a portal which automatically uses you Windows (AD) account to login, and sometime you need to override this because you need to log in using a different account. If that's the case, do this:

For Internet Explorer 8:

• Open Internet Explorer and navigate to the website you need to log in to
• Go To Tools → Internet Options
• Go to Security and select the appropriate security zone (internet or intranet)
• Click on “Custom Level” and scroll all the way down in the pop-up window
• In the “User Authentication” section select “Prompt for Username and Password
• Save your settings and refresh the page for a login prompt