I own a 2-3 year old HP Pavillion p6180t. It has a "Pegatron" motherboard with an Intel Core 2 Quad Q8300. About six months ago, it went from purr to 747 and applications like SpeedFan and Speccy revealed not only high fan speeds, but very high CPU temperatures.
I went to Fry's to pick up an aftermarket fan to find that most of today's common aftermarket fans, even when correctly picked to match the right cpu and socket, do not match HP's "screw in" fan type motherboards.
I called CoolerMaster who makes many of these fans and they directed me to a specific, relatively inexpensive fan that would work.
So that came a few days later, and it had cpu heatsync paste already applied to it.
But here's the real kicker, when I removed the old fan from the CPU, what I discovered was dried, cracked, seemingly useless cpu paste. After I cleaned that off, and put on the new cpu fan, the computer immediately quieted back down to a purr.
So, you might need a new cpu fan. But you might only need fresh cpu heatsync paste. And replacing the stock cpu fan with the appropriate coolermaster fan is not very difficult and quite effective.
That's my data point from owning an HP desktop.
3It was probably the dust keeping the sound deafened. LOL. – surfasb – 2011-09-18T15:54:48.423
2@surfasb The dust was not keeping the fans from not making noise as OP says but the fans are just as loud as they used to be – Alpine – 2011-09-18T15:59:57.313
2Your options are limited on an off-the-shelf PC like that. HP, Dell, etc. make most of the innards proprietary, so it's difficult to swap power supplies, fans, etc. for quieter or more efficient options. This is why some people build their own PC's; you can put in whatever parts you want, and if you want quiet and cool, it's fairly easy to accomplish. – bwall – 2011-09-18T16:04:06.420