I have this exact same problem. Brand new machine. Running 64 bit Win 7 Pro with 8 gigs of RAM.. The fan keeps coming on and turning off at random. Usually only stays on for a few seconds then shuts off. It is very annoying and frustrating. Called tech support. They replaced the motherboard and the fan onsite. Still is doing the same thing. It will do this all day. Without me even doing anything right after a reboot. No reason it should be doing this.
It has to be some kind of bios update needed, or fan firmware or something right. Chipset.. something.
C'mon Dell help us out here, before my little beauty of a laptop blows up.. or I throw it against the wall.