There are certain scenarios in which you can follow the steps suggested in the other answers, verify that Execution Policy is set correctly, and still have your scripts fail. If this happens to you, you are probably on a 64-bit machine with both 32-bit and 64-bit versions of PowerShell, and the failure is happening on the version that doesn't have Execution Policy set. The setting does not apply to both versions, so you have to explicitly set it twice.
Look in your Windows directory for System32 and SysWOW64.
Repeat these steps for each directory:
Check the current setting for ExecutionPolicy:
Get-ExecutionPolicy -List
Set the ExecutionPolicy for the level and scope you want, for example:
Set-ExecutionPolicy -Scope LocalMachine Unrestricted
Note that you may need to run PowerShell as administrator depending on the scope you are trying to set the policy for.
You can read a lot more here: Running Windows PowerShell Scripts