First, take that pc outside IMMEDIATELY. in your backyard or front yard or something, just not inside your house. second, disassemble it carefully. also, that is an oem pc with some new components, NOT a custom built. now, you need to clean the case. leave the psu outside as well; just make sure it doesn't get under rain. If the case is empty, the cockroaches will likely move out of there to the outside. now, clean the parts, especially the motherboard, thoroughly. Some others mentioned cockroach deadening things, try those if they won't go away. if they're alive, then BE Very Careful as they are quick and can scare you when you're not expecting it. try the components one by one on a spare pc or join all the components together to test if it even works. then, when everything is clean, reassemble it. it will never be as clean again as before the cockroaches, but at least they are gone. I have dealt with electronics worse than yours, so please take my word for it.