PC shuts down after a few seconds.

Hello everyone,
I'm having some problems with my old PC and I have no good solutions to fix it.

Here's what happened: For a few months now, my PC failed to save BIOS settings. Almost everytime I had to do the settings again. BIOS settings were going on Default everytime.
So today I replaced the CMOS battery with one from a old laptop. Meanwhile, I unpluged the IDE bus from CD-ROM and pluged back.
After that I turned my PC on again and after a few seconds it shuted down. There isn't a fixed time but it goes from 1-30 seconds.
Tried to rebuild the PC from 0 but the result is the same. I even replaced the laptop battery with the first one/ started PC without CD-ROM.
Can it be a short or something?
Thanks!