Computer "Sleep Mode"

I am operating a Dell Inspiron 530 Computer with Vista OS.

When my computer is in "Sleep Mode" it is automatically returning to "run mode" without any action from me. What could cause this to happen?

Prior to this problem starting I replaced my Internet Security program with an updated version. Could something in the new version of my internet security program be causing this to happen.

Prior to this time my computer would remain in "Sleep Mode" until I took action to return to normal operation.