On Windows 7, you can enable TFS automatic login by adding in a windows credential in Control Panel -> User Accounts -> Manage your credentials.
In the Credential Manager window, under the Windows Credentials section, click the “Add a Windows Credential” link to add one for your tfs server.
If your TFS Server is in another domain, you might also have to add the server to your local intranet in your internet settings. In IE, Tools->Internet Options->Security tab, click on the “Local Intranet” icon, and then the Site -> Advanced button
Now you have stopped TFS prompting for passwords all the time.
On Windows XP, you can find the credentials option in Control Panel -> User Accounts -> Advanced -> Manage Passwords