Windows Defender  is a free program that helps protect your computer against pop-ups, slow performance, and security threats caused by spyware and other unwanted software. It features Real-Time Protection, a monitoring system that recommends actions against spyware when it’s detected and minimizes interruptions and helps you stay productive.
This guide explains a few basics about Windows Defender and point you two a few important guides that’ll help you get the most out of Windows Defender.
Installing Windows Defender
Windows Defender is part of Windows Vista but it can also be installed  on Windows XP SP2 or later. Windows Defender can’t be installed on earlier versions of Windows than XP SP2. You can install Counterspy  which uses the same technology as Windows Defender but works on most versions of Windows as an alternative to Windows Defender.
Note: The company that makes Counterspy is a partner of GIANT AntiSpyware (the company bought by Microsoft a few years ago and eventually became Windows Defender) so it’s safe to use but will cost you a few dollars.
If you purchased a third party antivirus package like Symantec, McAfee, or Trend Micro, chances are that Windows Defender has been disabled. You’ll have to enable Windows Defender  before you can use it.
Scanning Your Computer
By default Windows Defender will scan your computer for malware once a day at 2am and automatically remove severe threats. Medium and low threats will not be removed automatically, but it’ll notify you and wait for you to tell it what to do. Windows Defender also protects your computer in real-time from pop-ups and malware.
You can manually perform a partial, full, or custom scan  if you feel the need.
If Windows Defender finds potentially malware you’ll be asked how to deal with them .
Using Software Explorer
If your computer seems sluggish Windows Defender has a tool called Software Explorer  that shows you detailed information about running programs. You can also use Software Explorer to stop programs from starting when Windows boots to speed up your computer’s boot time and overall speed.
Tweak Windows Defender’s Settings
Windows Defender is configured to work best for most people right out of the box. If you want to tweak Windows Defender check out my Windows Defender options guide .