<s>[INST] How to run multiple python version on Windows I had two versions of Python installed in my machine (versions 2.6 and 2.5). I want to run 2.6 for one project and 2.5 for another. 
How can I specify which I want to use?
I am working on Windows XP SP2.
 [/INST] Running a different copy of Python is as easy as starting the correct executable. You mention that you've started a python instance, from the command line, by simply typing python. 
What this does under Windows, is to trawl the %PATH% environment variable, checking for an executable, either batch file (.bat), command file (.cmd) or some other executable to run, that matches the name given. When it finds the correct file to run, it does it.
Now, if you've installed two python versions 2.5 and 2.6, the path will have both of their directories in it, something like PATH=c:\python\2.5;c:\python\2.6 but Windows will stop examining the path when it finds a match.
What you really need to do is to explicitly call one or both of the applications, such as c:\python\2.5\python.exe or c:\python\2.6\python.exe.
The other alternative is to create a shortcut to the respective python.exe calling one of them python25 and the other python26; you can then simply run python25 on your command line.
 </s>