msvc build actually needs Bison 2.2 or later, not 2.1. Or 1.875 as before.

doc/src/sgml/install-win32.sgml

-<!-- $PostgreSQL: pgsql/doc/src/sgml/install-win32.sgml,v 1.33 2007/03/17 14:30:00 mha Exp $ -->
+<!-- $PostgreSQL: pgsql/doc/src/sgml/install-win32.sgml,v 1.34 2007/03/17 17:11:41 mha Exp $ -->
 
 <chapter id="install-win32">
  <title>Installation on <productname>Windows</productname></title>
@@ -100,8 +100,8 @@
       <productname>Flex</productname></term>
      <listitem><para>
       Bison and Flex are required to build from CVS, but not required when
-      building from a release file. Note that Bison version 2.0 will not
-      work, but both earlier and later versions do. Bison and Flex can be
+      building from a release file. Note that only Bison 1.875 or versions
+      2.2 and later will work. Bison and Flex can be
       downloaded from <ulink url="http://gnuwin32.sourceforge.net"></>.
      </para></listitem>
     </varlistentry>

src/tools/msvc/pgbison.bat

 @echo off
-REM $PostgreSQL: pgsql/src/tools/msvc/pgbison.bat,v 1.5 2007/03/17 14:01:01 mha Exp $
+REM $PostgreSQL: pgsql/src/tools/msvc/pgbison.bat,v 1.6 2007/03/17 17:11:41 mha Exp $
 
 if exist src\tools\msvc\buildenv.bat call src\tools\msvc\buildenv.bat
 
 SET BV=
 for /F "tokens=4 usebackq" %%f in (`bison -V`) do if "!BV!"=="" SET BV=%%f
 if "%BV%"=="" goto novarexp
-if %BV% LSS 1.875 goto nobison
-if %BV% EQU 2.1 goto nobison
+if %BV% EQU 1.875 goto bisonok
+if %BV% GEQ 2.2 goto bisonok
+goto nobison
+:bisonok
 
 if "%1" == "src\backend\parser\gram.y" call :generate %1 src\backend\parser\gram.c src\include\parser\parse.h
 if "%1" == "src\backend\bootstrap\bootparse.y" call :generate %1 src\backend\bootstrap\bootparse.c src\backend\bootstrap\bootstrap_tokens.h