How does 'pip' locate python executable?

I tried installing some packages using this command:

pip install -r requirements.txt

However, this fails with this error message:

Fatal error in launcher: Unable to create process using '"c:\dropbox\tools\python\python.exe" "D:\Dropbox\Tools\Python\Scripts\pip.exe" install -r requirements.txt': The system cannot find the file specified.

Now, pip, and python, are both installed in D:\dropbox\etc, the exact same path as above, just on the D: drive.

I've checked environment variables to see if there's anything odd:

> set | grep -i python
Path=D:\Dropbox\Tool...;D:\Dropbox\Tools\Python;D:\Dropbox\Tools\Python\Scripts;...
PYTHONHOME=D:\Dropbox\Tools\Python
PYTHON_HOME=D:\Dropbox\Tools\Python
PYTHONPATH=D:\Dropbox\Tools\Python\Lib;D:\Dropbox\Tools\Python\Libs;D:\Dropbox\Tools\Python\DLLs

There are no environment variables referencing C:... for python:

set | grep -i "C:\\dropbox\\tools\\python"

I did not install Python on this machine, it was synchronized onto it using Dropbox, but I have initialization scripts for the console application that I use (Cmder) that set up the path to python and everything according to the dropbox location.

If I try to use python to run pip, it runs successfully:

python -m pip install -r requirements.txt
(bunch of package checks, nothing installed though)

My question is as follows: Where/what/how does pip calculate/find python executable location? Meaning, what should I look for in order to find that C: reference that it incorrectly uses?

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