The first on is a two year old gateway notebook (Intel Centrino)which started failing to power up a few weeks ago. The owner would 'dismantle' it, reassemble it and it would come back to life. This repeated several times. Now it won't power up at all. The power light comes on an nothing else after that. No monitor activity, the heard drive can be heard spinning but that's all.

The second is a six month old Positivo notebook (Brazilian brand) with a Core Duo processor. It has no history of tampering. It's supposed to be under warranty but I noted that the processor bay cover had been opened, possibly because the owner suspected processor failure.
This machine does not POST. The power LED lights up, the HDD can be heard spinning but there is no activity after that.

The owner says he has at least 5 colleagues with notebooks behaving the same way.

What could be the problem?

The Gateway seems to be a case of a poor contact/dry solder on the motherboard. Hence the past behaviour. Now the contact isn't making at all.

The Barzilian laptops (6 of them) sounds like a common manufacturing fault. Few of us are going to be any help here for the Positivo (Negativo).