On Windows, I normally just use the binary installer, but I would like to install NumPy only in a virtualenv this time, so I created a virtual env:
virtualen
Since pip can only install from source, you need to have the setup to build and compile NumPy to be able to use pip. I never tried this.
What I did recently to get NumPy, SciPy and Matplotlib into a virtual environment on Windows 7 is to copy the directories and egg files from my regular Python install directly into the virtual environment. For Matplotlib I also managed to unzip the binary installer directly into the virtualenv (I only tried this for the Gohlke installer for Python 3.2).
I did manage in the past to easy_install NumPy into a virtualenv from a binary distribution, but I didn't manage this time (NumPy 1.5.1).
Someone recommended http://pypi.python.org/pypi/pyg which should allow to install from binary packages to a virtualenv, but I haven't tried it yet.