First suggestion is to make sure your case is 100% contaminent free. Dust or Particles kill bearings in fans.
Second thing, if HP didn't use standard fans in the case, then you are pretty much SOL. You'll have to replace the case (and maybe the PSU as well). The fact that you could replace the Mobo is a good indication that they used standard fans.
Next is you should get http://www.cpuid.com/hwmonitor.php
which should display the RPM rates of your fans. For each case fan, you should record the RPM and the size of the fan.
Finally, to replace the fans you have with something quieter, go to your vendor of choice (eg: Newegg) and look for fans that use FBD (Fluid Dynamic Bearing) as the bearing type. These should be as quiet as you get get. What you want to do is find the fans that are physically the same size and have as close to the same RPM as you currently have as these will move the same amount of air and provide the same amount of cooling.
However, because they use an Fluid Dynamic Bearing, the dB they produce should be much, much lower than whatever you have now.