Fires created by desktop computers are mostly urban myths (only Lithium Ion batteries that short can cause fires in laptops). Smoking can occur if power connectors are shorted, as the wires can melt the shielding to create smoke. That electronic "burning" smell, the most dreaded of all smells for PCs, is overheated and possibly shorted ICs, resistors, capacitors, transformers, diodes, etc. But this is not an outright "fire".
A normal desktop or high-end gaming system cannot create a fire unless someone tampered with the PSU and added flammable material to the mix. The 18AWG and 24AWG wires used for power supply cables will spark, and melt apart and smolder (creating smoke) before they ever have a chance to catch fire.