Professional and Ultimate editions of Windows come with a built in Remote Desktop (RDP) feature that allows you to access your machine remotely while away from home or the office. Unfortunately, it is limited by default to one concurrent user per session, this is not a technical limitation but rather a licensing one.
Case in point, Remote Desktop in server editions of Windows by default supports two concurrent connections to remotely troubleshoot or administer a computer. However, there are a few reasons why concurrent sessions would come in handy for power users not necessarily running a server. For example, if you have a dedicated Media Center PC running in the living room, you'll be able to remotely access all files on the machine without interrupting the person watching TV.
Or if you are sharing a computer with other users, concurrent Remote Desktop sessions will allow more than one person use that system under a different or even the same user account, without kicking each other off. By patching a file called termsrv.dll, located in %SystemRoot%\System32\, this is possible in all editions of Windows 7, Windows Vista and Windows XP.