Your computer's power supply is the single most important component in the case. But it's also the most boring, so when it comes time to skimp on costs, the power supply is often a generic or budget brand. Sometimes this is okay. But often the power supply acts erratic, causes restarts and errors because other components are not getting the consistent voltage they require. If you are experiencing problems you cannot quickly isolate, be sure to test the power supply and rule it out before spending too much money.
Tips on Troubleshooting a Power Supply
Most families have a other computers of similar size. Before you spend money on a tester or a new power supply, you might try just swapping in a power supply you know works. That is the low-tech solution, but it often takes a lot of extra effor that people aren't willing to expend.
Buy a Power Supply Tester
You can buy a power supply tester for relatively cheap. While a multimeter is a better overall solution because you can use it for other things, a power supply tester is a fast and easy way to tell if the PSU is good.
Check For Proper Voltage
Check the voltage setting on the back of the power supply. While this is not likely the problem, just confirm it is set correctly for your region. If the computer was recently moved, it's possible the switch was moved. 115v is typical for the United States.
Make Sure Power Supply Fan is Working
If you have an older PC, you should make sure the fan on the power supply is working. If it's not, you need to replace the power supply immediately. You can sometimes blow them out with compressed air, but if it is not spinning at all, the power supply must be replaced.
Is the Power Supply Powerful Enough?
Lastly, you should make sure your power supply is rated high enough for your needs. If you built the system a few years ago, you have probably added extra hard drives, a more powerful graphics card and maybe some new RAM. Make sure your power supply has enough voltage to handle it all.