1. If you are running MOSS 2007 on Windows Server 2003 the you have to download and install PowerShell from http://support.microsoft.com/kb/968930. Windows Server 2008  natively supports PowerShell.
  2. Set the  Execution Policy to 'RemoteSigned'. You can check it by running 'Get-ExecutionPolicy' cmdlet
    The default execution policy for PowerShell is “Restricted” (commands only, not scripts)
         
# All scripts running locally are allowed
Set-ExecutionPolicy RemoteSigned

This means downloaded scripts must be signed by a trusted publisher before they can be run.

     3. You need to load the SharePoint assemblies with the following command

#    Load the SharePoint assemblies
[Void][System.Reflection.Assembly]::LoadWithPartialName("Microsoft.SharePoint")

A bunch of PowerShell scripts are available at http://www.powershell.nu/2009/09/08/moss-2007-script-collection