Is a login box prompt all of a sudden coming up every time you attempt to browse your Sharepoint site, did it previously work okay and works from a different machine with the same user credentials?
If so you could be experiencing a similar problem I have come across with cached credentials.
On Windows 7 browse to:
Control Panel\User Accounts and Family Safety\Credential Manager
Under the ‘Windows Credentials‘ heading remove the affected users login details that appear for your Sharepoint server.
Try browsing to your Sharepoint site and the login box should not appear.
Thanks to this article for pointing me in the right direction, it also contains other common issues/fixes relating to the login box appearing: