No longer POSTing / no VGA signal
I'm trying to fix my sisters machine, which has stopped POST-ing (no post beeps, no VGA signal, but everything powers up / fans spin). The machine is just older than a year old. My sister reports that it only did it once every now and again, and it's become more and more common. I booted it up once since I've look at it but haven't been able to since.
I think I've narrowed it down to either motherboard or PSU. If I remove the RAM, the motherboard beeps constantly. I tried replacing with other RAM that I have, and the same problem occurred (so it's not the RAM). I tried removing the CMOS battery for 15-20 minutes and then replacing it, no change.
The motherboard is a cheap MSI All-in-One, and the PSU is a generic (numan) 450 watter. I have no spares for either of them, so I can't test to see which one is causing the problem. What else can I try?