I found a solution which I hope it will work for you.
I have tried everything written in this post and python still was not working into my arcgis; giving the message 000574 error Python is not installed.
I have Win7 64 bit, and I was using Arcgis 9 then, installed Arcgis 10 and then, upgraded to 10.2 last three years, and in my last installation Python was not found by Arcgis. Also it was not working from python27.exe or pythonw.exe when I tried to run them from the folder C:\Python\Arcgis10.2.
However, I went at another computer where Arcgis 10.2 was recently installed and Python were running well. It was a Win7 32 bits but all was running fine. Then, I have checked out that python27.dll in my computer was different size (and also the creation date) than the python.dll installed in the other computer. Also, the date of the python.dll in my computer was older than the date I had installed Arcgis10.2. So, I discovered it was not replaced or updated with the last arcgis installations.
Then, I decided to remove the older python.dll file from my computer (located into System32 folder and syswow64 folder) and replace them with the newer file installed in the other computer. Also, I have pasted another copy of the new python.dll file into syswow32 just in case.
Everything started to work immediately when I opened ArcGis. I hope this could help to some of you.