1. Through Unknown AD's
The internet is f**** up. sometimes a message pops-up saying "your computer has been infected" install this antivirus. Come on!. If you have an anti-virus software why again trying other anti-virus . Sorry for being rude, okay such type of ads are normally having bad intention towards your pc. for safety just ignore them . and enjoy your day
2. From email
Some how email act as our Internet ID, even sometimes our billing receipt details from online stores are being saved there. And I can say the internet is not safe place but trying to make it safe, is our task to do it.
Your computer can get a virus by opening unknown links from emails or downloading attachments from unknown sender. So to be safe avoid opening emails from unknown sender
3.During installation of software from the internet
During installation of any softwares from the internet, there is always an option for quick installation custom installation and automatic installation. And when you select automatic installation, the program might allow another simple software and sometimes a virus, spyware or adware . so the best way to install is by selecting quick installation. And carefully read the info's.
4. From usb and other secondary storage devices
if you plug in infected secondary storage device, then its easy for virus to sneak in your pc. But to be safe, make sure you scan the device after plugin.
5. Downloading software's from untrusted sites
But it seems that we trust pirate sites nowadays, but we should understand the risks of such sites. am not gonna talk more about pirate sites. So for I, I normally read the setup information of software's I want to install, reading reviews from others.