LOL, is that the conduit that covers what appears to be a SAS controller and that SAS cable?
@Spud
A SOFT bristle tooth brush (very very soft) with pure rubbing alcohol and the power from a system completely drained, as well as all batteries on the system remove is about the best for getting any tar like substance off of a board. You just have to ensure there is NO electricity in anything at all, and let it FULLY dry, as well as be very gentle. That aside, a microfiber cloth works wonders on the rest of the dust with fans and the case it self.
Honestly don't need rubbing alcohol unless there is a tar or some sort of oily substance on the boards/fans/case that is causing the dust to stick heavily. Just a spray of air and a microfiber cloth that's been soaked and wringed out of rubbing alcohol does wonders. The rubbing alcohol keeps the cloth from being capable of ESD.
It really comes down to, what are you cleaning, and what is on it that your trying to remove. In my case it was cigarette tar and dust, which you HAVE to pretty much scrub off and use rubbing alcohol.
Regular dust? Canned air, and a moistened cloth (using alcohol to moisten it) to clean large surfaces, canned air for the boards, cloth for fans/metal.
Oily substance on the board? Same as cigarette tar/dust.
Being honest... At our shop we sold a spray foam for cleaning electronics... All it was is a foam that contained alcohol. We actually kept bottles upon bottles of rubbing alcohol on hand to clean really nasty systems, and sold the spray foam at a premium to people that wanted the "proper" stuff.