Usually when you work with TFS which is in another domain or if you are working on a Virtual Machine with its own domain Visual Studio 2010 keeps asking you for password every time you want to open a TFS based project.
This solution works on Windows Server 2008 and Windows 7:
Go to Control Panel -> User Accounts and Family Safety -> Credential Manager -> Add Windows Credential and enter the address and your credentials as following:
For Window Server 2003 and XP you can use the following:
Source of the XP Solution: HowTo: Teach Visual Studio to Remember Your TFS Password