So then I guess you don't use the internet right? Because every time another program (Not limited to just web browsers) accesses the internet, it has some authority to run and download other files. Not all viruses are .exe files, and many types of malware can infiltrate your computer simply by visiting a webpage. Every time you "view" a web page, your web browser is downloading files to your hard drive. Whenever Windows, Adobe, your web browser, Java, or games download updates, there is a potential to be hacked. Programs are not written perfectly, people can use exploits to distribute viruses and obtain personal information without you knowing about it. Every time you install security updates you have to give administrative permission to do so, but not installing the updates makes it even easier to be hacked, so you can't ignore them Whenever a file not originating from the PC is written to the hard drive (even from CDs and flash drives, or memory cards), there is a potential for malware, spyware, or viruses. In fact, through exploits using flash drives and other physical media, it is possible to become infected without ever being connected to the internet at all. Because people are constantly finding loopholes through programs like web browsers, Flash, and product updates (Even "safe" products), common sense alone will not prevent viruses. In fact, common sense says install an anti-virus, something you aren't doing, correct? If you really need to squeeze every bit of performance out of your computer, either upgrade it or find a minimal antivirus program. There are plenty out there that do not hog resources, not ever antivirus acts like Norton.