More 0.3.21 release prep

Bump version number in Windows installer file
Update READMEs.

doc/README_windows.txt

@@ -0,0 +1,34 @@
+Bitcoin 0.3.21 BETA
+
+Copyright (c) 2009-2011 Bitcoin Developers
+Distributed under the MIT/X11 software license, see the accompanying
+file license.txt or http://www.opensource.org/licenses/mit-license.php.
+This product includes software developed by the OpenSSL Project for use in
+the OpenSSL Toolkit (http://www.openssl.org/).  This product includes
+cryptographic software written by Eric Young (eay@cryptsoft.com).
+
+
+Intro
+-----
+Bitcoin is a free open source peer-to-peer electronic cash system that is
+completely decentralized, without the need for a central server or trusted
+parties.  Users hold the crypto keys to their own money and transact directly
+with each other, with the help of a P2P network to check for double-spending.
+
+
+Setup
+-----
+Unpack the files into a directory and run bitcoin.exe.
+
+If you have Microsoft Security Essentials, you need to add bitcoin.exe to its
+"Excluded processes" list.  Microsoft Security Essentials->Settings tab,
+select Excluded processes, press Add, select bitcoin.exe, OK, Save changes.
+
+The software automatically finds other nodes to connect to.  You can
+enable Universal Plug and Play using a menu entry or set your firewall
+to forward port 8333 (TCP) to your computer so you can receive
+incoming connections.  Bitcoin work without incoming connections,
+but allowing incoming connections helps the Bitcoin network.
+
+See the bitcoin wiki at:  https://en.bitcoin.it/wiki/Main_Page
+for more help and information.