Amazing how much grime gets sucked into these constantly running little vacuum cleaners computer cases. At work I roll our server cart out into the shop and blow huge clouds of dust out of the cabinets with compressed shop air annually. The desktop clients get the same treatment whenever the case needs to be opened up, or when they need more than superficial maintenance. Doesn't take care of smoke or any other aerosol which adheres to the interior surfaces, but we don't have that problem and this clears out the dust bunnies and cobwebs nicely, keeps things working cool and makes everything much more pleasant and less problematic to work on. Haven't had to do any more extensive internal cleaning than that on the computers, but I'd use CRD QD or an equivalent if I did.
I've used CRD QD on some of our industrial equipment circuit boards and other electrical components out in the shop that gets nastier oily buildup and it works great. Off topic, but to clean greasy, grungy hydraulic and mechanical components, automotive brake cleaner works really well and leaves no residue, but I wouldn't trust the hydrocarbon solvents in it on electronics.