The three sources to consider are vibration, cooling fan noise and hard drive noise. So the first obvious thing is physical isolation. I.E. dont set the mics on a surface (desk, table) directly connected to the computer.
Some cooling systems are quieter than others, some very quiet and very expensive. Most Hard drives arent real loud anymore, but its still a concern. Of course SSD's are noiseless.
The first thing to do is consider proximity of course, and also barriers of some sort can help, especially if acoustically absorbing in nature. Enclosures arent a good option though as they negatively effect cooling. I would consider an acoustic "curtain" (similar to the curved sheilds for recording vocals), but on a larger scale between the CPU and mics as a possible solution. Also for other ideas, look at the sheilds used to attenuate drums or guitar amps levels, though those tend to be more reflective than absorbing.
I think in terms of isolating the mic from the noise as much (or more) than isolating the noise from the mic. Be sure to always be aware and consider the pattern and direction of your mic positioning. "Aiming" the mic away from the source of unwanted noise (and any surfaces that are going to be first order reflective of the noise you are trying to abate). Close-micing the desired source and correct gain staging can help a great deal.
If a very quiet environment is desired, such as in a well treated room and recording of particularly low sources, you may want to consider long cables and a remote CPU installation and/or using a control surface for basic transport functions while recording.
I am designing a space where the CPU is on the other side of a well insulated wall, and I am considering a bluetooth keyboard test (not certain of the range), as well as a USB run to a basic control surface. We'll see how that evolves.........