I tried to upgrade my PC with a CPU change. The motherboard is a Gigabyte P35-DS3L, the old CPU is Core 2 Duo E6550, the new one is a Core 2 Quad Q9400.
I had to to take out the graphics card and two of my four ram bars to reach the mounting of the CPU cooler, but all in all the change was no big problem.
Now, when I switch on the PC, it is powering, but nothing happens on screen. Sometimes it seems to get stuck in boot loop, sometimes it just sits there and does nothing and sometimes it switches completely off after some boot tries.
So I decided to put in the old CPU again and look, if I have to do a BIOS update. And now I have the same problem with the old CPU.
My next step will be to rip all apart and clean everything and then build it up from scratch to make sure, everything is in the right place and nothing is loose.
Any other ideas, what could be causing this problem?
Edit Problem solved. I reset the BIOS as suggested by @FrankThomas and then updated the BIOS as suggested by @Ramhound. Now I'm rocking on four cores. I will see, if the power supply is good enough for the new setup. Thank you guys for all the comments.