i've installed py 2.7 (64bit) on my PC with Win7 (64bit) without problem but I'm not able to run *.py scripts via DOS shell without declare python full path.
Let me better explain :
If I type D:\ myscript.py it doesn't work. The script is open with wordpad 
If I type D:\ C:\Python27 myscript.py it works and run correctly
I try to change the default application software for *.py file via Win7 GUI ( control pannel etc etc) but without success.
Python is not present in the list of available sw and in any case also with the manual set I'm not able to associate python.exe at *.py files.
I've checked in my environment variables but I've not  found problem (python path  is declared in Path = C:\Python27\;C:\Python27\Scripts).
I've tried also to modify HKEY_CLASSES_ROOT->Applications->python.exe->shell->open->command :
old register  value "C:\Python27\python.exe" "%1"
new register  value "C:\Python27\python.exe" "%1" %*
without success.
Any suggestion?
Thanks
@slv 's answer is good and helped me a bit with solving this problem. Anyhow, since I had previous installations of Python before this error occured for me, I might have to add something to this. One of the main problems hereby was that the directory of my python-installation changed.
So, I opened regedit.exe and followed these to steps:

I searched the entire registry for .py, .pyw, .pyx and .pyc (hopefully I did not forget to mention any here). Then, I radically deleted all occurrences I could find.
I searched the entire registry for my old python-installation-path (e.g. C:\Users\Desktop\Anaconda3). Then I replaced this path with my new installation path (e.g. C:\Users\Desktop\Miniconda3). Thereby, I also came across and replaced HKEY_CLASSES_ROOT\Applications\python.exe\shell\open\command which @slv mentioned.

Afterwards, it was possible again to connect a .py-file from the Open with...-menu with my python.exe.
Impossible to set python.exe to *.py scripts on Win7