Most of the time, it just annoying. I'm not talking about those serious ones, i've got adware, browser hijacker and ransomware, drive me crazy already. Since the infection, my PC running speed was decelerated and it got weirder later, as the system resources have been taken by unknown applications/processes running in the BG (triggered by viruses). Have to remove the computer virus completely and timely.
Computer virus can slow down your computer. Might corrupt
your system files and make some programs faulty or corrupt. Virus damage your
boot sector creating problems when you boot into the windows and it steal
important information from your computer and send to some other person. Might
change the power ratings of your computer and could blast the system. So, to protect your PC have to install antivirus scanner
Ransomware is one of the most "popular" malware these days.
Once it's taken over your computer, threatens you with harm, usually by denying you access to your data, which means all the files on your computer would be locked with an encrypted key. The attacker demands a ransom from the victim, promising — not always truthfully — to restore access to the data upon payment.
The victim would be shown instructions for how to pay a fee to get the decryption key. The costs can range from a few hundred dollars to thousands, payable to cybercriminals in Bitcoin mostly.
There are a number of vectors ransomware can take to access a computer. One of the most common delivery systems is phishing spam — attachments that come to the victim in an email, being faked as a file they should trust. Once they're downloaded and opened, they can take over the victim's computer, especially if they have built-in social engineering tools that trick users into allowing administrative access. Some other, more aggressive forms of ransomware, like Dharma Ransomware, exploit security holes to infect computers without needing to trick users.