Vista conserving power

Hi, my computer keeps going in to sleep mode to conserve power when I'm not using it at first glance this would be wonderful, but I am downloading large files that take much more than 2 hours to complete, which means when I leave it alone and come back it appears that my computer is turned off but when I push the power button the screen turns on and I'm directed to the login screen. After I login to my account my downloads have been terminated. There must be an easy way to fix this, say the screen saver options.