Is this a new security feature or is my computer haunted?

If my wireless Logitech keyboard is moved out of range of the reciever, my
computer will automatically lock and then go to sleep until I move the
keyboard back in range... I am running a standard cheapie Logitech wireless
combo on Vista RC2. My mobo is a Gigabyte K8U-939 with 2GB DDR and an Athlon
64 4200+. Video card is an AGP nVidia 6800 Ultra.

I can't seem to find any info about this "problem" anywhere and it really
freaked me out the first few times it happened. I thought my computer could
somehow tell I stood up from the chair and secured itself for my absence.

Thanks for any help!