GUID = Globally Unique IDentifier: http://www.wired.com/news/technology/0,1282,18405,00.html and http://support.microsoft.com/support/kb/articles/q176/7/90.asp.
MAC = Media Access Control address. Each and every Ethernet device (e.g., network adapter, port on a hub) has a unique MAC address, which is part of the hardware. In Windows 9x/Me Start, Run, winipcfg will display the MAC address (Adapter address). ipconfig /all (from DOS prompt or Start, Run ipconfig/all) will show it in Win NT/2000/XP.