Computer Shut off problems

I have three computers that are shutting of randomly.

1. dell laptop
1. toshiba laptop
1. dell gx270 desktop

1. The dell laptop shuts off before i can finish the express diagnostics test. It has scanned clean for viruses.

2. Toshiba laptop runs for a while and then shuts off. Once that happens it takes a while before it will start up again.

3. Dell desktop has similar problems to the dell laptop.


All computers are owned by different people so its not my power supply that is causing the problem. Any suggestions as to how to go about diagnosing this problem would be greatly helpful.

 

Three things to check or do first

PSU
Temperatures
Clean

 

for the laptops, maybe you could take out the batteries and run on ac power only to see if that still happens