Is it possible to quieten a PC to be as silent as a Mac or laptop?

Make sure all components have enough space and that the heat has enough room for escaping out of the case (for example with cooling-paste)

Probably not. There are some ridiculously expensive PCs that ship with liquid cooling, but from my experience, Mac desktops and laptops are consistently much quieter than PC counterparts.

As ibnabouna implies it depends how much money you want to spend and I’d also add what needs you have for your system. If you’re into top level gaming hardware then it’s going to be expensive to get things quiet. e.g. water cooling.

However if your needs are more moderate then buying a fanless motherboard and video card will reduce a lot of the noise (small fans are the worst). I also found one of the quiet power supplies made a big difference.

Here are a couple of links that you’ll find useful:

http://quietpc.com/
http://www.hushtechnologies.net/

I’ve bought several items from quite pc and have been happy with their service. Hush has always been a little bit more than I wanted to spend.