How do I backup my wallet? Using bitcoin-qt, multibit, or any of the others, how do I backup and save my wallet in case I need to reinstall Windows?
bitcoin-qt


open the File menu
click Backup Wallet...
select a file to save the backup as


MultiBit

From http://multibit.org/help_backupWalletUsingPrivateKeys.html:


  You can export your private keys from a MultiBit wallet into a file
  (using the 'Tools | Export private keys' option) and, as long as you
  do not manually create new receiving addresses, you can use the
  private key file as a backup for the wallet.


Armory

From http://bitcoinarmory.com/index.php/start-page/what-is-armory/features:


  Not only are the wallets determinstic, but there is a built-in dialog
  for printing permanent copies of your wallet.  When you first create
  your wallet, make sure your printer is on and click the “Paper Backup”
  checkbox.


Electrum

From http://electrum.org/:


  You do not need to perform regular backups, because your wallet can be
  recovered from a secret phrase that you can memorize or write on
  paper.