For whatever reason there are hardly any GTD programs for Windows that also happen to be freeware. Most have disappeared or moved to payware models. I've only been able to find one promising candidate.
ThinkingRock....their homepage is here:http://www.thinkingrock.com.au/
However, they have moved to a payware model with v3.x of their software, which is the only version they offer on their homepage. The last freeware version, v2.2.1, can be found here:http://sourceforge.n...ojects/thinkingrock/
The good news is its crosss-platform. Windows, Mac OS X, and Linux are supported. The bad news is it achieves this by being written in Java. Some people don't like Java touching their systems. I respect that.
I do like their licensing terms, though, for the paid version. $39 buys a license that lets you use the program on multiple computers with any mix of platforms, free upgrades, and free iOS and Android mobile apps.
Standard disclaimer: I've never used this software. I've never heard of this program or the company behind it until today. I've never used a GTD task-prioritizing system in my life.