I am getting error message displayed when I try to write to/read from an excel file

[Vishvesh Pai]

File "C:\Python24\Ex celread.py", line 6, in -toplevel-
excel = Dispatch("Excel .Application")
File "C:\Python24\Li b\site-packages\win32c om\client\__ini t__.py", line 95, in Dispatch
dispatch, userName = dynamic._GetGoo dDispatchAndUse rName(dispatch, userName,clsctx )
File "C:\Python24\Li b\site-packages\win32c om\client\dynam ic.py", line 98, in _GetGoodDispatc hAndUserName
return (_GetGoodDispat ch(IDispatch, clsctx), userName)
File "C:\Python24\Li b\site-packages\win32c om\client\dynam ic.py", line 78, in _GetGoodDispatc h
IDispatch = pythoncom.CoCre ateInstance(IDi spatch, None, clsctx, pythoncom.IID_I Dispatch)
com_error: (-2147221005, 'Invalid class string', None, None)

why this error is displayed?