How to remote logon to domain user account

I've got a VPN newly set up, and from home I can view my files on my home directory and all files in shared directories on my office servers.  I can't figure out how to actually log on to my office network account and operate as if I were sitting at my office desk.  Can I do that natively with windows, or do I need some 3rd party product like LogMeIn or GoToMyPC?

Office setup: Windows 2003 Server.  Home setup.  Windows 2003 Server and three  PCs with Vista Ultimate, XP Pro, and Win2K.

Thanks, Ron