Look at the MB's capacitors. Older machines had inferior caps that will blow. Start removing components and power up. Remove RAM sticks one at a time, disconnect drives, PCI cards, etc. If the machine powers up with a component removed, done. Otherwise it's probably the motherboard. If so, be prepared to replace the MB, processor, and RAM in one fell swoop.
I have an emachines t2042 with a celeron processor running windows xp. One day it started turning itself on and making clicking/static sounds from the speakers. Also had hot smell from the tower. I replaced the power supply and now can only turn power off with the switch on the power supply and get nothing to screen or keyboard. The monitor does have power and message says monitor is work check pc. I suspect motherboard problem.Any advice would be appreciated. Thak you. Joe