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To reproduce:
1) Download and run Python 2.7.9rc1 Windows installer.
2) Select "Install just for me."
3) Enable "Add python.exe to Path."
=>
Expected: Current user PATH edited.
Actual: System PATH edited.

The "just for me" option isn't actually a per-user install, it actually just restricts use of the core Python DLL to Python itself and not any other applications that may want to use it (by putting the DLL in the Python folder instead of the system folder). Basically everything else about this option is a bug that I consider it of scope for 2.7 at this point. If someone else figures out how to fix it, I can try and review the patch, but I'm not intending to figure out the install script well enough to fix it.
For what it's worth, the Python 3.5 installer will have a true per-user option (no admin privileges, no shared files with other users, per-user environment and file associations, etc.)

For me this isn't too high priority. I typically install Python for all users on Windows anyway, and recommend that also for my clients. I just tested this RC to see how (ensure)pip works in practice, decided to install only for the current user, was very happy to notice that there's an option to set PATH, and then decided to submit an issue when I noticed the system PATH was updated.
Because setting PATH is opt-in, and because 2.7 installer apparently requires admin rights and thus always can updated system PATH, I doubt this problem is going to actually matter to anyone. Great to hear 3.5 will have real "only for me" option.

Irrelevant since version 3.5.