Here's what you need to use sa-compile on Windows:
A compiler. See BuildSpamcOnWindowsForFree to get a free one from Microsoft.
re2c. Download the Windows binary from http://sourceforge.net/projects/re2c and extract it into your /perl/bin folder.
For SpamAssassin 3.2.0, you need a modified sa-compile. See http://issues.apache.org/SpamAssassin/show_bug.cgi?id=5447 to get it. Copy into into your installation directory, then re-make and install SA.
You need to create a make.bat in your /perl/bin folder that calls nmake.exe. It should look like this:
nmake %1 %2 %3 %4 %5 %6 %7 %8 %9
You need to make sure the environment variables are set prior to calling sa-compile. This is probably easiest done by creating a new batch file to run it. Create sa-compile-run.bat in /perl/bin like this:
call "C:\Program Files\Microsoft Visual Studio .NET 2003\Vc7\bin\vcvars32.bat" call sa-compile.bat