I have a desktop computer and if I'm on for like a 1/2 hour or more my tower just shuts itself off. I used spray air and cleaned out the openings on the side, but not sure what it could be. Is my tower crapping out??

I would check your fan/fans.

Many times, it is overheating, that causes it to shut down.

Perhaps the fan is not running properly, or the temp sensor that tells the fan when to turn on may be bad as well.

Agree with above post, see if you can open can and spray out fan and confirm fan is working

Here is link for freeware cpu temp monitor
Download SpeedFan Temperature Monitor 5.0 - SpeedFan Temperature Monitor is a free widget that uses SpeedFan - Softpedia (http://www.softpedia.com/get/Windows-Widgets/System-Utilities/SpeedFan-Temperature-Monitor.shtml)

Do you have to power the system back on when it shuts down or does it automatically come on and go back into windows? If it goes back into windows then it is probably an issue with windows. If you have to manually turn it back on you are probably looking at either a Motherboard or a Power Supply. What brand of system is it?

I have a desktop computer and if I'm on for like a 1/2 hour or more my tower just shuts itself off. I used spray air and cleaned out the openings on the side, but not sure what it could be. Is my tower crapping out???
It's an emachines computer from walmart. I have to turn the whole thing back on again and the system starts back up from the windows start up

Try the blowing out the system with a can of compressed air and see if that helps. Make sure all of the fans you see are spinning. When it shuts off see if the power button on the system has a light in it and if the light is solid and what color it is.