This tip is suitable only if you are running Windows XP, 2000, 2003 Server, MCE 2005. Windows Vista and 7 do not include this feature - look here for more info: http://windowslive.com/Connect/Post/E8E5CC039D51E3DB-2123982
Windows Recovery console is a tool that helps you to recover your OS from serious errors. In fact, it's a small operating system from Microsoft (but incapable of running any programs, except several built-in commands). It is available from some installation CDs (when booting from them), but as well you can always install it on a hard drive - indo a hidden folder, cmdcons, on a boot partition.
To install it, press Win+R, type cmd, press ok, then type X:\i386\winnt32 /cmdcons (replace X with actual letter of your optical drive). After installation, a new option appears in your boot menu - it's a Recovery console. If a problem occures, you can use it to fix the issues.