DELL DIMENSION 5100

Sounds like the wiring connecting the power switch pins on the motherboard and the power button might have issues.

You could try the following at your own risk:

Take a flat head screwdriver and make a bridge between the two motherboard pins and the screwdriver until the computer turns on (should be instant). Immediately take the screwdriver out when the computer boots up.

If that works, then the wiring has an issue and needs to be replaced or fixed. In my opinion, it sounds like the wire coating might have a tear in it where it is worn down to the metal and is consequently rubbing against the case or other components causing the bizarre behavior.

Hope that helps!

Before taking everything out of the case, you might test a few things along the way - try disconnecting all unnecessary components and see if you can get to bios - disconnect the HD, optical drives, mouse, and any USB peripherals except keyboard - leave only CPU, HSF, ONE stick of memory, keyboard, and video card with monitor connected. For the disconnected components, make sure you disconnect both PSU and cables to mobo. See if you can get to bios.
If you can, add back components one at a time to isolate the problem, starting with the memory, then HD (at this stage shouild be able to boot to windows) , mouse, and USB devices.
If not, disconnect video card and keyboard and see if it at least appears to recognize memory. If still not - then take out of case.