Windows default shell and path not found

i've changed the Windows default shell with my application and everythings is ok. My aplication is in c# and i need to run some external python scripts, and here is the error because the Path variable is not found.

I've tried to set the Path for the started process :

  process.StartInfo.EnvironmentVariables["PATH"] = "c:\\Python27;c:\\Python27\\Scripts";

This just dosen't work.

The problem is that the PATH variable is not loaded because i've changed the default windows shell so explorer.exe dosent come up.

Thank you.