Not that I know of.....you can do alot with some relatively inexpensive mods though.....the three worst noisemakers are the case fan, CPU fan, and power supply.
Case fans - Zalman, Papst, Rasurbo, Nexus. I have a Panasonic Panaflo in my DAW and really like it.
CPU fans - Zalman, Panasonic, Papst, Coolermaster, Vantec, Thermaltake. I have a Vantec on mine.
Power supply - this one is usually the most expensive of the three mods.
Zalman, Coolermaster, Vantec, Enermax. I use an Enermax.....it has 2 fans and is VERY quiet. I can barely hear my DAW when it's on, but it was really loud before I modded it.
Here are a few websites you might look at:
http://www.endpcnoise.com
http://www.quietpc.com
http://www.compuquiet.com
I know you were asking about an off-the-shelf quiet PC.....it's kind of hit and miss on them, mostly miss. Most people are not overly concerned with PC noise unless it's really loud, so therefore most manufacturers don't concentrate on noise reduction. It would be better to mod it yourself, and it's pretty easy. The CPU fan is the hardest of the three.
Hope this helps