Using ArcPy to identify Shapefiles instead of os.walk() via *.shp file extension?

I am trying to write a script that identifies all Shapefiles within my folder structure and write a list of the .shp files with their full path file name to a newly created .txt file. I was able to get this code to work with PNG files but when i try to search for .shp files i get a RuntimeError. I have every idea this is because there are multiple files that make up a shapefile.

Is there a way to do this using ArcPy?

His is my code so far:

import os, arcpy

StartDir = "C:/Temp/"
OutputFileName = "CarolinasSHPs"
tempName = StartDir + OutputFileName + ".txt"
outFile = open(tempName, 'w')

for root, dirs, files in os.walk(StartDir):
    for f in files:
        arcpy.env.workspace = root
        if f.endswith(".shp"):
            outFile.write( root + f + 'n' )

outFile.close()