This is what we do to clean computers that have tabacco/tar/nicotine smells and stains.
Look for Air Freshener that have Neutrador, and use it around the said unit, this will be cleansed the internal smells, but if the unit have been used in such an environment for a long time, I would really suggest getting the fans looked at, as accumulation of sticky tar (death by smoking?) on the fans is not uncommon.(usually overheating and noisy fans)
I personally use Air Freshener from Amway as this also works in neutralizing the smells. For cleaning the body and keyboard, a mix of Liquid Organic Cleaner, also from Amway (1 part LOC and 5 parts water) a damp cloth, and a soft tooth brush, helps with the body, keyboards, and screen, as this will gently remove stains and smell without damaging any parts of the computer.
Been using them for 10 years to clean computers that smells and have stains!