I think we have a few knee jerk reactions here. Without knowing what is in your rig, how much natural ventillation you have in your cab, and how the case is situated, it is hard to say if you need a fan. If you are running an Athlon 1200 say, and have a PSU that has multiple fans, and you have vents in the cab that provide enough heat release, then you should be fine.
If you need to check, depending on the motherboard, there might be a temp reporting program, like Asus Probe. If not, I'm sure the motherboard has a temperature sensor in the bios...just run a bunch of games, get the processor nice and hot, then do a warm boot and check the temp in the bios.
As far as the hardrives are concerned, again it depends on the rig. If you have 1 harddrive in the box with nothing in the slots above and below, then you should have plenty of heat transfer to keep them running for a long time. When you hear about water cooling and other cooling solutions, they are typically used in over-clocked boxes, well outside the manufacturer's recommendations or very top end machines.
In a nut shell, a low end system with proper natural ventillation should not require case fans.