From OpenFOAMWiki

OpenFOAM doesn't run on Microsoft Windows out of the box because it relies on a case-sensitive file system. With some work however it is possible to run OpenFOAM on Windows.

Note: None of these solutions are officially supported by the OpenFOAM Foundation.

1 Do it yourself

2 Commercial solutions

3 Community solutions

Sorted by date of the last update:

blueCFD-Core is an open source project provided by blueCAPE, aiming towards providing OpenFOAM technology natively on Windows, through the consolidation of community porting efforts. They provide installers for Windows.
Provided OpenFOAM versions: OpenFOAM 4.x, but also plan to release several other versions and support.
Last Update: August 2016
Caelus-CML is a fork of OpenFOAM 2.1.1 and was designed to make it easier for new users. They provide 64-bit installers for Windows, Ubuntu and RHEL.
Provided OpenFOAM versions: a fork of OpenFOAM 2.1.1, but their own development line continues with validation and verification procedures.
Last Update: April 2016
This project provides the modified source code of OpenFOAM, as well as binary installation with a limited feature-set from blueCFD-Core. It has now been replaced by the latest generation of blueCFD-Core as an open source project.
Provided OpenFOAM versions: 2.0.x and 2.1.x, both 32-bit and 64-bit. No MPI support.
Last Update: 3rd of April 2013
OpenFOAM-2.1.x for Win64
Based on Symscape's instructions, but built with Open-MPI instead of MS-MPI. Support thread for this build available here: Windows (x64) Binaries for OpenFOAM-2.1.x using OpenMPI
Last Update: 7th of January, 2013
OpenFOAM 1.7.0 for Windows 32 bit
Based on instructions from this wiki page. Support thread for this build available here: OpenFOAM-1.7.0 for Win32
Last Update: 9th of December, 2010
openfoam16wi32 project at SourceForge
On Yuu Kasuga's blog (in Japanese) is a short tutorial on how to install and use the files on this project, which provides OpenFOAM 1.6 32-bit binaries for Windows.
Last Update: 9th of September, 2010 (for the RAR file; 7zip file on 10th of June, 2013)
Provides a port of OpenFOAM 1.5, built natively on Windows. It also provides a GUI for editing case files and launching ParaView. It seems to have been a proof-of-concept project for a master thesis.
Last Update: 28th of May, 2009