I'm trying to use python in powershell

Try setting the path this way:

 $env:path="$env:Path;C:\Python27"

For what's worth, this command did it for me (Python3.3) :

[System.Environment]::SetEnvironmentVariable("PATH", $Env:Path + ";C:\Python33", "Machine")

I just had to restart the Powershell after that.

$env:path="$env:Path;C:\Python27" will only set it for the current session. Next time you open Powershell, you will have to do the same thing again.

The [Environment]::SetEnvironmentVariable() is the right way, and it would have set your PATH environment variable permanently. You just have to start Powershell again to see the effect in this case.