How can I make my desktop PC cooler and quieter?

This is an excellent although slightly outdated guide.

First thing you should do – check what your warranty allows you to do. Then check if the components in your pc are compatible with retail ones (power connectors, MB mounting sockets, etc)

Second, Locate the source of noise.

Case fans: Easy to replace and mount. Just take any silent 3 pin or 4 pin on the market with the desired size and replace. Keep the airflow direction the same. Slow 120 mm fans have amazing airflow while very quiet. Vibration absorbing pads are welcome.

CPU fan: Again the market is saturated with high quality and performance coolers. If your MB is standard and has the proper mounting holes just buy the one that best fits in your budget and noise requirements. Vibration absorbing pads are welcome.

PSU fan: Don't know here. It could be replaceable but there will be some (dis)assembly required. Otherwise - replace the PSU – again a lot of excellent choices on the market.

VGA cooler: There are aftermarket solutions, but I cannot comment – usually when my vga shrieks a lot, I cannot hear it because of the sounds the zombies make when chainsawed.

Hard drives: Using SSD for a system drive and some normal hard drives for slow media data is very quiet solution, although expensive. There are also noise dampening solutions for hard drives.

Case: it could emit sounds if the case is of very poor build quality or has some loose screws, dust or cables chaffing fans.

Edit: One of the best sources for everything quiet.

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.