Datorn startar inte (fungerar ibland)

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Greetings!

My parents have a computer that’s about four years old. It has worked fine up until now, and I need some help sorting out what’s wrong with it.

The computer has some serious trouble booting up. It runs for a few seconds (5-10) and then it just stops. It doesn’t matter if the computer has been on for three days and I just try to reboot or if it hasn’t been on for a few days. Once it does start however, it runs flawlessly for days if need be, but every once in a while you need to reboot anyway, and then it usually doesn’t start. Annoying to say the least.

About six months ago I bought a new graphics card for it, and when I put it in I found out that the card needed 400w to run, and sadly the computer only had a puny 250w power supply. It worked anyway, though on low settings, I don’t know if this has anything to do with the problem though as it worked fine until now.

My uneducated guess would be that it’s either the power supply or the motherboard that’s giving up. My questions are:

1. Is there any way to test if one (or both) of them are broken?
2. If I must change the motherboard, how do I know which socket the CPU needs?

I’m going to my parents this weekend, so I can check exactly what hardware is in the computer then if you need more info, so these specs are straight from memory:
Pentium 4, 2.0Ghz
512Mb Ram
80Gb hard drive
Gainward 256Mb Graphics card
250w Power Supply
Windows XP Home Edition

It’s an old computer, but it works well enough for my parents and can run most games at moderate settings. The power supply needs to be changed either way, but I don’t want to order anything until I’ve got some other peoples view on the problem.

Thanks for Your time
Cyne

Köpte ett Gainward GeForce 6600GT 256MB GDDR3, AGP8X, "Ultra/1980" Tv-out,DVI, Retail

Funderar nu på att köpa ett FSP 400W nätagg. Har fått bra recensioner på SweClockers, kostar <500 kr och borde räcka datorns livslängd ut (Funderar även på att köpa samma nätagg till mig och min flickvänn, + nya grafikkort)

Låter det som en vettig plan att beställa ett nytt nätagg och prova, och om det fortfarande inte löser problemen, skaffa ett nytt moderkort?

Inga andra förslag på vad som kan vara på tok?

Tack för alla svar!