This works for me 50% of the time when repairing friends's PCs, starts with taking the computer outside, take off all the covers.
+Make sure the dehydrator is working properly, turn on the compressor along side it. Adjust your compressor's regulator to 80Psi, [so you don't "blowout" something] snake the nose into all the cavities and blow out all the dust thats accumulated.
+Unlatch the ram chips from their slots, blow clean, replace.
+Unlatch the video card, unplug the video card fan, blow air into the fan orifice till nothing comes out, plug the fan back in, plug the video card in.
+Unplug the hard drives, noting which SATA goes with which. Blow air into the hard drive cavities, reconnect hard drive, repeat for the other drives: this works for A:/ , B:/ , and D:/ also.
+Find a way to blow a stream of air behind the motherboard
+Detach the CPU heatsink, blow that out, reinstall
+Disconnect all case fans, clean off the blades, reconnect
+Blow all the dust out of the PSU, if you think you've got all of it try blowing in a different orifice.
As a precaution, keep the PC sitting on the driveway for a bit to eliminate any wayward condensate. A full clean-out like this takes me about 30 minutes if I have all the tools. I didn't even start on the software part yet. I'm more of a hardware guy, so even if I wasn't able to find anything software wrong with the PC, at minimum I get credit for the cleanup.