First of all, make sure you are not running too many programs at a time. Multitasking puts massive burden on the RAM and as a result, the machine gets slow automatically. Turn off all the unnecessary programs.
Go to the ‘Start’ menu and enter the Control Panel.
The Vista user will find the icon of Windows Defender right in front. You will need to double-click to run it.
Push the ‘Scan’ button and this will quickly start a total system scan with Defender.
The scan might take several minutes, depending on the speed of your computer. Once it is done, click ‘Tools’ and deselect ‘Options’.
You will find the box labelled "Automatically scan my computer" right under the heading of automatic scanning. If it is already checked, directly select times for scans by using drop-down menus.
Scroll down and make sure that the "Use Windows Defender" box is checked. The final step is to click on ‘Save’. Close the window now. Your Windows Defender is up and running now.