Every computer connected to the internet is exposed to a steady stream of files, links, email attachments and downloads — and some of them are malicious. Antivirus software is your first line of defense. Today the terms “antivirus” and “antispyware” usually describe the same product: one program that protects against viruses, spyware, ransomware and other malicious software.
How antivirus protects you
- Real-time protection. The program checks files as they’re downloaded, opened or run, and blocks anything dangerous before it can do harm. If you see a warning, choose the recommended action — usually Quarantine or Remove — and then run a full scan.
- Definitions. Antivirus keeps a constantly updated list of known malicious software, called definitions — think of it as a “cheat sheet” of known threats. When a new threat appears, the maker releases new definitions and your program downloads them automatically, often several times a day.
- Behavior and cloud checks. Modern antivirus also watches what programs do and checks suspicious files against the maker’s online database. That lets it catch brand-new threats that aren’t on the cheat sheet yet.
- Scheduled scans. A full scan of the whole computer finds anything that slipped past real-time protection. Once a week is a good rhythm — schedule it for a time the computer is on but you’re not busy using it.
Do you need to buy antivirus?
For most home users, no. Windows 10 and Windows 11 include Microsoft Defender Antivirus at no extra cost. It protects against viruses, spyware and ransomware, and it updates automatically through Windows Update. Windows also includes Microsoft Defender SmartScreen, which warns you about dangerous websites and downloads, and a built-in firewall. Together — with Windows kept up to date — they provide solid baseline protection.
Businesses often benefit from a paid business-grade (“endpoint protection”) product. These add central management, so every computer’s protection can be monitored from one place, along with extra defenses against ransomware.
Whichever you use, what matters most is that it’s switched on and up to date. A paid product whose subscription has expired may stop receiving updates, leaving you less protected than the free built-in tools would. To check, open Windows Security and look at Virus & threat protection.
If you still have Kaspersky installed: the U.S. government banned Kaspersky security products in 2024, and the software stopped receiving updates for U.S. customers in September 2024. Out-of-date protection isn’t protection — replace it.
Why one antivirus is enough
It’s tempting to think two antivirus programs are better than one. They aren’t. Running two real-time antivirus programs on the same computer causes problems:
- Slowdowns. Each program uses memory and processing power, and two can slow a computer to a crawl.
- Conflicts. The programs can mistake each other for threats, causing false alarms, freezes or crashes.
- Damage. One program may quarantine the other’s files, breaking it and making it hard to repair or remove.
- Extra cost. Two subscriptions cost twice as much for no added protection.
Windows normally turns off Defender’s real-time protection automatically when you install another antivirus product. One well-chosen, up-to-date product, plus a firewall and good habits, is the right setup.
Signs your computer may be infected
- Frequent pop-ups, unfamiliar toolbars or strange notifications.
- New programs or icons you didn’t install.
- Your browser’s home page or search engine changes, or websites redirect to places you didn’t choose.
- The computer becomes unusually slow, freezes or crashes during everyday tasks.
- Your antivirus has been switched off and you didn’t do it.
- Friends or customers receive strange emails or messages that appear to come from you.
What to do if you suspect an infection
- If you see a ransom message or your files suddenly won’t open, disconnect the computer from the internet (unplug the network cable or turn off Wi-Fi) and get help before doing anything else.
- Update your antivirus, then run a full scan. It can take anywhere from 30 minutes to several hours — let it finish.
- Follow its instructions to quarantine or remove what it finds, and restart if prompted.
- Scan again after the restart to confirm the computer is clean.
- If the problem keeps coming back, run a Microsoft Defender Offline scan: open Windows Security → Virus & threat protection → Scan options. The computer restarts and scans before Windows loads, which catches threats that hide while Windows is running.
- Check what starts automatically: open Task Manager (Ctrl + Shift + Esc) and look at Startup apps for anything you don’t recognize.
- Change your passwords — starting with email and banking — from a different computer or phone that you know is clean, and turn on two-step verification where it’s offered.
- If the infection can’t be removed, restoring from a recent backup or reinstalling Windows may be the safest option.
Habits that matter as much as antivirus
- Keep Windows and all your programs updated — turn on automatic updates.
- Be cautious with links and attachments, even from people you know. If a message is unexpected, check with the sender another way.
- Check the web address, not just the padlock. Scam sites use “https” and the padlock too; what matters is that the address is the real company’s.
- Keep the Windows firewall (or another reputable firewall) turned on.
- Turn on two-step verification for your email, banking and other important accounts.
- Back up your important files regularly, and occasionally check that you can restore them.
Need help?
If your computer is acting up, PSA can find and remove the problem — residential virus removal is a $100 flat rate. We can also help you choose and set up the right protection for your home or business. Call (707) 506-6802 or send us a message.
This article replaces PSA-0004 (2017) and PSA-0030 (2025).