Dear Pamela ,
Check for your Firewall Settings, there is a Setting in firewall which Automatically Disconnect the Internet if you are not using it for some Time.
If your firewall is having any Problem then try to reset it.
Check for your DSL Modem , when your Internet Disconnects Automatically,you will have Lights Blinking in your modem such as
Power,DSL,Internet,in the mean time If the Lights Corresponding to DSL / Internet is in Red-color,and Blinking then the Problem will be with your ISP.
If the Lights are not Blinking and Steady with Green Color When your internet is Disconnected, then i am very much sure that there is a Problem with your Firewall.
Any way can you Just let me know how do you use the Internet, Wired or Wireless , Cable or 3G.
Mac OS - Windows Xp