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You may find OpenJDK 6 and 7 binaries for Windows in openjdk-unofficial-builds github project.
Update: OpenJDK 8 and 11 LTS binaries for Windows x86_64 can be found in ojdkbuild github project.
Disclaimer: I've built them myself.
Update (2019): OpenJDK Updates Project Builds for 8 and 11 are available now.
In case you are still looking for a Windows build of OpenJDK, Azul Systems launched the Zulu product line last fall. The Zulu distribution of OpenJDK is built and tested on Windows and Linux. We posted the OpenJDK 8 version this week, though OpenJDK 7 and 6 are both available too. The following URL leads to you free downloads, the Zulu community forum, and other details: http://www.azulsystems.com/products/zulu These are binary downloads, so you do not need to build OpenJDK from scratch to use them.
I can attest that building OpenJDK 6 for Windows was not a trivial exercise. Of the six different platforms we've built (OpenJDK6, OpenJDK7, and OpenJDK8, each for Windows and Linux) for x64 so far, the Windows OpenJDK6 build took by far the most effort to wring out items that didn't work on Windows, or would not pass the Technical Compatibility Kit test protocol for Java SE 6 "as is."
Disclaimer: I am the Product Manager for Zulu. You can review my Zulu release notices here: https://support.azulsystems.com/hc/communities/public/topics/200063190-Zulu-Releases I hope this helps.