Hi 01042,
Welcome to the PCHF!
I think that from what you are describing there might be a few problems to look at.
Firstly, did you also swap the power supply? The
PSU from the old pc might not be able to run the newer system properly.
Secondly, try a USB Keyboard and mouse as the PS2 port might be faulty.
Thirdly, try getting the system up and running with only what you really need i.e. Floppy, cdrom, hard drive, vga card. Ideally test the hardware that you can on another working pc to make sure it's ok.
Fourth, Make sure that everything is firmly in place on the board, memory is a classic.
Let us know how you do