You usually have to preface a command with "python" so your computer knows what to interpret the script with:
Code:
python "C:\Documents and Settings\user\Desktop\tab2opf.py" -utf "C:\Documents and Settings\user\My Documents\angpol.tab"
But if that were the problem, the error would likely be different (unknown command/program or some-such).
It looks like tab2opf.py is a python 2.x.x script. You aren't, by chance, trying to run it with Python 3 are you? (If you just type "python" at a command prompt, you should be able to determine your Python version). That won't work.