My son recently gave me a computer after he bought a new Mac Laptop.  It is a pretty decent machine with a Quad core processor (3.3 MHz). I have decided to built it up into a second machine.
 
It was very dirty inside and full of dust so I cleaned all that out fine.  In order to really clean the CPU heatsink and fan I unscrewed the fan from the top of the heatsink.  Then  (after releasing the CPU heatsink) the whole heatsink came away from the CPU core itself.  I noticed that there was little thermal grease present and what was there was quite dry.
 
I did a test.  Before I did anything I booted up into the bios and I noticed the CPU temp was around 53 deg C which I thought was rather high especially as it was not doing anything much.  Just running a new install of Win 7 that is all.
 
I decided while the CPU was all in pieces to clean off the old thermal grease and re apply some new thermal grease.  This grease or paste is very important as it transfers heat from the CPU core into the heatsink itself that gets cooled by the fan.
 
After doing all this and re assembling the CPU temp has dropped well down to under 40 deg C or even at 37 deg most of the time now in idle mode.  I felt this was quite a significant improvement.
 
I know many are not savvy to doing all this but it is worth mentioning and perhaps checking either yourself or take it into a qualified repair person.
 
Check you CPU temp in the bios and do some research into what is normal for your CPU.  If you feel your CPU is running a little hot it may due the reasons above.
 
Cleaning the dust out of your computer is a good thing to do on a regular basis.  Also the thermal grease or paste won't last forever and over time must deteriorate.  Another good reason for NOT leaving computers on 24/7.  (dust and the thermal grease drying out)

I use special cleaner by the company that makes Arctic Silver heat sink compound.  It is a two step process, the first step cleans off the old heat sink compound and the second step prepares (purifies) the surface for the new compound.
 
http://www.newegg.com/Product/Product.aspx?Item=N82E16835100010
 
I also use the Arctic Silver heat sink compound.

With a Noctua cpu cooler sitting on my cpu, I occasionally get messages from my cpu letting me know it's cold, please turn up the heat...

gustabo
With a Noctua cpu cooler sitting on my cpu, I occasionally get messages from my cpu letting me know it's cold, please turn up the heat...

+1
I have the D14 and it's an ice box. :))
 
Also, I leave my PC on 24/7 (per the DAW builders suggestion)......but do clean out all the dust bunny's inside on a monthly basis. 

Should also mention that i7 chips tend to run a bit hotter than i3/i5. Just the nature of the beast. If you're overly concerned about it, an aftermarket CPU fan might be the way to go though I've always found the stock fans to be more than sufficient if you're not overclocking.

TerraSin
Should also mention that i7 chips tend to run a bit hotter than i3/i5. Just the nature of the beast. If you're overly concerned about it, an aftermarket CPU fan might be the way to go though I've always found the stock fans to be more than sufficient if you're not overclocking.

The i7 Ivy Bridge CPU's are known to run hotter and when I built my current machine, the recommendations here as well as other forums were to actually get an aftermarket cooler as the stock cooler wasn't up to par in keeping it cool enough. 
Of course, I also planned on overclocking (and do overclock) my system......so that may have had some weight in their suggestions on using an aftermarket cooler. 

I have an i7 3770 on my DAW system.  I'm using a Thermaltake Water 3.0 Pro liquid cooler.  Very good and quiet.

I've an Ivy Bridge and with a big Noctua cooler and a 140mm fan at 5 volts it's within thermal spec even after 15 minutes at 100%. Only other fan in the PC is another 140mm pushing air out of the top back of the case. The psu and graphics card are fanless and everything stays cool enough.

Giving the internals a good dusting out every so often helps though. As does trying putting the cpu fan beneath the cooler blowing outwards rather than on top blowing down towards the cpu. My current DAW is built that way and not only is cooling better than with the fan on top or blowing sideways but there seems to be much less dust buildup between the cooler vanes than in otjer PCs I've built which had the fan working the "conventional" way. PC cooling is worth experimenting with if you build your own or modify an off the shelf machine. Sometimes the conventional fan positions are far from optimal for noise or cooling.

It has to be said that more recent motherboards seem much better designed for quiet heat dispersion. The little high speed fans that used to be stuck on motherboard chips were dreadful noise generators. Component and PC manufacturers also seem to be paying some attention to keeping noise down these days (at last).

The hottest running cpu I ever had was a P4. Those things really drew some watts and gave of a great deal of those watts as heat. The Ivy Bridge is cool running in comparison. The noisiest PC I've ever heard was an off the shelf gaming machine a friend's son bought. Translucent case, everything lit up with LED chains inside and (count 'em) 14 90mm fans going flat out. Sounded like an airliner starting down the runway and it still ran hot. The case design was simply dreadful for airflow.

I clean mine out with a device that the students at work christened "Derpy Duster" after gluing googly eyes on it. It works well, just blast the dust out periodically. I recommend them highly.
 
As for thermal compound, breakdown isn't a big problem if you use quality stuff. Arctic Cooling MX-4 is a good choice. If you buy a prefab system, well then it varies how good the grease is. It'll usually outlast the warrantied life of the system no problem though.

Thanks Sycraft......I've been thinking of what to use to clean out the fans inside....this might do the trick. How long does "Derpy" generally last?