arcpy - Changing which python ArcGIS uses?

I have a couple of python installations on my windows 7 machine. I have the one that was installed by ArcGIS, in a folder called "ArcGISx6410.2", and another I recently installed myself directly from the python website, I've put that in a folder called "DirectDownload".

The one installed by ArcGIS is the default one which I normally use, and which the windows path points too. This is what I get when I call python from the command prompt:

Microsoft Windows [Version 6.1.7601]
Copyright (c) 2009 Microsoft Corporation.  All rights reserved.

U:\>python
Python 2.7.5 (default, May 15 2013, 22:44:16) [MSC v.1500 64 bit (AMD64)] on win 32
Type "help", "copyright", "credits" or "license" for more information.
>>> import sys
>>> sys.executable
'C:\\Python27\\ArcGISx6410.2\\python.exe'

But, ArcGIS itself appears to call the "DirectDownload" installation, and that one doesn't contain numpy, and can therefore not import arcpy. I don't need the "DirectDownload" installation any more, but if I remove it (or rename it) then ArcGIS will not load at all, it will not get past the splash screen.

How can I tell ArcGIS to use it's own installation of python, like it was doing quite happily until I installed the other version?