A few things to consider:
Ventilation Holes ( PSU, GPU exhaust, etc.)
Gunk from cigarette smoke can clog up the ventilation holes, which can cause the computer to overheat.
When tar from cigarette smoke builds up on a CPU fan, it can keep the fan from spinning properly, which can make it overheat.
Smoke can damage a hard drive by getting stuck between a platter and the head, or by damaging the motor. Can greatly reduce the life of an HDD by as much...