Replace cmos battery with a brand new one and clear cmos settings via motherboard jumper.
I bought this dell OptiPlex off eBay some months ago (you can see about how long ago it was from my first post). The fans are just on full speed. The power indicator light is turning on and off in the pattern of on off on, off for a bit longer, on, off, on, then off again.
OptiPlex 7020 SFF, fans full, power light flashing