I have this power supply I bought today and I can't get to know which is his watt power: on the label it says 145W, but also 250W, so I'm a little bit confused.
Here's an image
And I have another problem, too: this PS only has ONE 20 PIN connector and my MB also needs the 4 PIN one, so that I'm not sure if I can use it in my main PC.
Is it possibile to use this PS on my MB, even if it needs two power connectors?
Buy an adapter such as this: http://www.pcconnectionexpress.com/IPA/Shop/Product/Det.... Your power supply is 250w, so your total system load can't exceed 200w. Most power supplies can run at 80% of their rated capacity. If you are using any medium to high end video card, keep your compaq for testing only and get a better one, such as corsair or antec.
No. If you are using the onboard video, then it should work ok. If it doesn't work or is worn out (bad capacitor, etc.) then the board won't power up consistently. Motherboards also have capacitors that can wear out, so don't invest any more in your system, such as a ram or cpu upgrade. It's too old.
That power supply is ancient, the -5 V rail and the low 12 V rail current limit are give aways. The 8 amp limit on the 12 V rail means you have only 96 W for the CPU, GPU, and fans which likely isn't enough for a P4 system. The lowest power 2.8 GHz P4 chip requires 68 W for itself, some need up to 89 W.