PC won't boot: Psu or Mobo?
 

At the moment i have disconnected everything from the motherboard. There's only the cpu + cooler, memory and 1 fan

Now when I connect only the 20 Pin ATX cable with the Motherboard and i try to boot, the fan and the cpu fan begin to work.

However when i try to boot with the 12V connector (for the CPU) everything remains dead

with 12V connector do you mean the 3-pin fan connector, a 4-pin E-ATX connector or 8-pin EATX connector?

If it's one of the last two, get a hold of a second test PSU, if it's the first, try another fan header?

the 12V connector in my case is the 4 pins, needed to power to cpu (according to the mobo manual)

can you lend a PSU from a friend or nearby shop to check?

If the 4-pin connector isn't connected, does the computer boot or just sits there doing nothing ?
If you can't find a second psu, you could try measuring the voltages using a multimeter, anything between 11.4V and 12.6V on the 12V rail is acceptable.

What Psu and motherboard ?
We need info :)

There are some Gigabyte motherboard (P965) which have re-boot issues and disconnecting the 12V P4 cable seems to resolve the issue. This tuitorial on PSU connectors provides everything you need to know. Why there is a 4-pin 12V connector for additional power to the processor (known as P4 connector) and why some boards use an 8-pin 12V connector (known as EPS12V).

The photo below shows a Gigabyte board and the P4 CPU connector.