pc rebooting of its own accord

[almal]

my pc developed a tendency to simply reboot itself without warning. this escalated in frequency untill now it will not stay on for more than a few moments before doing it again. obviously this makes it impossible to do anything on it. that was on my desktop, but distressingly the problem seems to have developed on my new laptop too! they have never exchanged any data, so i dont think it could be from a virus, unless they haveboth independantly caught the same problem. can anyone shed any light on what might be the problem or things that i should look at to get more info on whats going on?

[mjc]

Some more info about both...Windows versions, make/models, etc...

For the desktop, open it up and give it a good cleaning...there may be dust build that is causing it to overheat.

The laptop could also be heat related, but it shouldn't require being opened to clean...just a can of compressed air and something to hold the fan still.