You could use Window's built-in Remote Desktop to connect to their computers, but unfortunately that requires their computers to have Windows 7 Pro/Ultimate, Windows Vista Business/Ult, or WinXP Pro. Only the host requires this though, Win7 HP can connect as a remote desktop client. There are hacks out there that enable Windows 7 Home Premium computers to be the remote desktop host though, but I'm not sure how much tomfoolery that requires.
You could also check out TeamViewer: http://www.teamviewer.com/en/index.aspx
or UltraVNC: http://www.uvnc.com/