Freeware a Systems Administrator can't live without.

• WireShark - Packet Sniffer
  
• HijackThis - For finding out what runs at startup and for removing spyware.
• Firefox - Best browser to avoid spyware.
• WSUS - Windows Server Update Services
• Memtest86+ - Great system memory tester.
• BartsPE - Preinstallation enviroment for fixing a computer that won't boot.