Until recently, a Windows version seemed unlikely...
But with beta version 2.1.0, GnuCash is now available in a Windows self-installing executable. I installed it on my wife's Windows laptop yesterday and it seems quite stable for a beta version.
The current stable version (2.0.5) can be installed on Mac OSX using the Fink installer (http://finkproject.org/) or on Debian Linux with "aptitude install gnucash gnucash-docs" (as root of course). GnuCash can also be installed on Ubuntu fairly easily ( http://www.ubuntugeek.com/install-gnucash-financi
GnuCash is a great free program with features that rival its ad-infested, monopoly-owned rivals. Why not try it out?
PS — It looks like now there is a complete suite of open source software that runs on both Windows and Linux. There is OpenOffice.org for an office suite (sans Outlook), Evolution (or Thunderbird with Lightning) for an Outlook replacement, Firefox for a web browser, the GIMP for photo editing, PidginIM for instant messaging (formerly called Gaim, but renamed to avoid a trademark dispute), and now GnuCash for accounting.
If you're thinking about switching to Linux, switching to these applications first could be a great way to prepare yourself and your data for the move."