If yo are using original Windows then you can have security essential for free. Most important thing is, what you are installing in your computer, i mean pirated copies or original softwares. If you are installing cracked software's then what ever the virus guard will not work. Because sometime in those cracked versions have hidden viruses inside the software, can not even detected by virus guard coz it goes to windows system folders and by doing that escape from virus guard scanning. So you think it's clean, but it's infected.