Prevent Automatic Logoff in windows vista / windows 7 (stop return to logon

After an idle period windows sends me to the user logon screen. My account is an admin account with no password set, so i simply hit the spacebar to return to windows. However, any downloads I have going get interrupted (i guess its turning off the NIC?) and I must start them over if its not a resumable server, etc. I dont think logout is part of my power management settings, so im guessing its a default security setting? I would like to disable it.

PS – i have no screensaver set so its not being done by screensaver security