Win Vista : Change Windows Defender settings:

Online protection.
Windows Defender.

Windows Defender helps automatically safeguard your computer against pop-
ups, slow performance, and security
threats caused by spyware and other unwanted software.
Change Windows Defender settings:
Click the Start button, type defend in the Search box, and click Windows Defender.
Click Ttools in the Windows Defender window.
Select Automatically scan
my computer to use
Windows Defender.
Make any adjustments to the schedule.
Check boxes for Check for updated definitions before scanning and Apply default actions to items detected during a scan.
Select any other options you want, and click Save.