Microsoft has integrated Windows Defender, the anti-spyware software, in Windows Vista and Windows 7, and while it is easy to disable Windows Defender, it is not really possible to uninstall Windows Defender.
Disable Windows Defender
To disable Windows Defender, open Windows Defender > Tools > Options > Uncheck Use Real Time Protection check box and also Use Windows Defender under Administrative Options check box. Click on Save.
Also type services in start menu search bar and hit Enter to open Services. Change the startup type of the Windows Defender Service from Automatic to Disabled.
Uninstall Windows Defender
Though I have not tried this personally, there is one way being recommended on the internet. It is said to have worked, for some. Try it only if you are keen to uninstall Windows Defender. Run the following from an elevated command prompt:
msiexec /uninstall windowsdefender.msi /quiet /log uninstall.log
I must add that I do not advise trying to uninstall Windows Defender, as uninstalling / deleting Windows Defender in Windows Vista is known to cause other irritants later on, since it is well-integrated with the OS.
Ported from WVC
In Windows 8, Windows Defender is far improved and now includes complete anti-malware protection.