numpy traceback error with ArcGIS Desktop 10.1

numpyerror.py

>>> import arcpy
Runtime error 
Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "c:\program files (x86)\arcgis\desktop10.1\arcpy\arcpy\__init__.py", line 24, in <module>
    from arcpy.toolbox import *
  File "c:\program files (x86)\arcgis\desktop10.1\arcpy\arcpy\toolbox.py", line 342, in <module>
    from management import Graph, GraphTemplate
  File "c:\program files (x86)\arcgis\desktop10.1\arcpy\arcpy\management.py", line 22, in <module>
    import _management
  File "c:\program files (x86)\arcgis\desktop10.1\arcpy\arcpy\_management.py", line 14, in <module>
    import _graph
  File "c:\program files (x86)\arcgis\desktop10.1\arcpy\arcpy\_graph.py", line 27, in <module>
    import numpy
ImportError: No module named numpy