So will it just not boot at all? No intro logos or bios menu's showing?
Can you take a look at the motherboard for me and tell me if there are any bulging/swollen/leaking capacitors (look like metal cyclinders). As I know some of the older Dell Optiplex's had a problem with this, but not neccesarily always a dell problem.
Heres an example of a bulging/swollen one. See how there 'bulging' out of the top. If they are leaking you will see the leakage ontop also.
What make and model is the PC? Is it custom built or made by someone like Dell??