I just built this system a few weeks ago, and have been using CoreTemp 1.0 to monitor the processor temp while in Windows. Up until this past weekend, the core temps have never been above 30 degrees C, now they are running 38 to low 40's. Im not gaming, and not overclocking but I am usually running a MS Office product, multiple tabs/windows in Mozilla, Media Player, etc at the same time. When I close everything out and open Task Manager, nothing is eating up CPU resources.
I am using a liquid cooling system- Antec Kuhler 920 with the coolant temp averaging 31 deg C (used to be 28). Does this need to get returned?
38c to 40c idle temperature is quite normal. My i7 is at that same temperature while writing this with nothing else on. You should not worry about anything unless your cpu reaches over 90c when doing intensive stuff.
Whats the ambient (room) temperature?
On a hot day (~30°C) my CPU will idle at around 40°C, regardless of the overkill amount of custom water-cooling I have on it.
With a good custom loop, you can expect idle temps to be ~10°C above ambient, with that I would expect closer to 15°C
Idle temperatures are of little concern. Run Prime95 Small FFT's for 10 minutes to determine your load temperatures. Post your results along with Ambient temperature, then we'll see if there's anything to be concerned about.