Malware has gotten harder to detect. Modern viruses don't crash your computer, they run quietly in the background, stealing data, hijacking your browser, and recruiting your machine into botnets. Knowing the warning signs can save you from a far worse outcome. Here are the seven most reliable indicators that something malicious is running on your PC, along with the immediate steps to take.
1. Dramatically Slow Performance
If your computer suddenly takes twice as long to boot, open applications lag, or basic tasks feel sluggish, that's a red flag, especially if nothing else has changed recently. Many types of malware run resource-intensive processes in the background: cryptominers use your CPU to generate cryptocurrency for an attacker, spyware constantly scans and transmits data, and some ransomware variants encrypt your files in the background before revealing themselves.
Before blaming malware, rule out mundane causes: too many startup programs, a nearly full hard drive, or a Windows update running in the background. Open Task Manager (Ctrl + Shift + Esc) and look at the CPU and Memory columns. If an unfamiliar process is consuming a significant percentage of your CPU when you're not doing anything, that warrants investigation.
Press Ctrl + Shift + Esc to open Task Manager. Sort processes by CPU or Memory usage. Right-click any suspicious process and choose "Search Online" to find out what it is.
2. Unexpected Pop-Ups and Ads
Pop-up advertisements that appear outside your browser, on the desktop, in the system tray, or as Windows notifications, are a classic sign of adware infection. Legitimate software doesn't do this. If you see ads appearing when your browser isn't even open, or if your browser suddenly displays ad banners on sites that were previously clean, a browser hijacker or adware extension has almost certainly been installed.
Also watch for fake "warning" pop-ups claiming your computer is infected and urging you to call a phone number or download software. These are scareware tactics, do not call the number and do not download anything they recommend.
3. Your Browser Homepage or Search Engine Changed
Did Chrome or Edge suddenly start opening a different homepage? Is your Google search being redirected through an unfamiliar engine? Browser hijackers are a common form of malware that modify your browser settings to drive traffic to ad-revenue sites. They often arrive bundled with free software, a video converter, PDF tool, or game downloaded from a sketchy site.
If you didn't manually change these settings, you have a browser hijacker. Check your browser extensions immediately: in Chrome, go to chrome://extensions. Remove anything you don't recognize or didn't intentionally install.
4. Your Antivirus Has Been Disabled
One of the first things sophisticated malware does is disable or interfere with your security software. If Windows Defender or your third-party antivirus shows as disabled and you didn't turn it off yourself, that's a major warning sign. Some malware also prevents you from re-enabling protection or blocks access to antivirus websites so you can't download a scanner.
If you can't run your antivirus at all, boot into Windows Safe Mode (hold Shift while clicking Restart, then navigate to Troubleshoot → Advanced Options → Startup Settings → Restart → press 4). In Safe Mode, most malware won't load, which allows your security tools to run more effectively.
If your antivirus is disabled and you can't re-enable it, do not continue using the computer for anything sensitive (banking, email, passwords). Power it down and call a technician.
5. Unusually High Network Activity at Idle
Malware frequently communicates with remote servers, uploading stolen data, receiving commands, or downloading additional malicious code. If your internet activity light is blinking rapidly when you're not using the computer, or if your network usage meter in Task Manager shows sustained activity while everything is closed, something may be sending or receiving data without your knowledge.
Check Task Manager → Performance tab → Open Resource Monitor, then click the Network tab. Look at the "Processes with Network Activity" section. Any unfamiliar process showing significant send/receive activity deserves investigation.
6. Programs You Didn't Install Appear
Malware frequently installs additional software, toolbars, utilities, or secondary programs that serve as additional attack vectors or revenue generators for the attacker. Go to Settings → Apps → Installed Apps (Windows 11) or Control Panel → Programs → Uninstall a Program (Windows 10). Sort by installation date and look for anything you don't recognize, especially items installed recently around the time problems started.
Be cautious about what you uninstall, some legitimate system components have cryptic names. Search anything you're unsure about before removing it.
7. Contacts Are Receiving Emails or Messages You Didn't Send
If friends or colleagues tell you they've received strange emails, spam, or phishing links from your address, your email account may have been compromised. Some malware harvests email credentials and uses your account to spread itself to everyone in your contact list. Check your email's Sent folder, if there are messages you didn't write, your account has been breached.
This can happen even without malware on your device; a data breach at an email provider can expose your credentials. But if combined with other symptoms on this list, it strongly suggests an active infection.
What to Do If You Suspect Malware
Disconnect from the internet
Pull the network cable or disable Wi-Fi. This stops any ongoing data exfiltration and cuts off the malware's communication with its control server.
Boot into Safe Mode
Safe Mode loads only essential Windows components. Most malware won't run in Safe Mode, making it much easier to detect and remove.
Run a full scan with Malwarebytes
Download Malwarebytes Free (from a clean device if your browser is compromised) and run a full scan. It catches many threats that Windows Defender misses.
Change passwords from a different device
Don't change passwords on the possibly-infected machine. Use your phone or another computer to update email, banking, and any accounts that may have been accessed.
Call a professional if the scan doesn't clear it
Some advanced rootkits and persistent malware require specialized tools and expertise to fully remove. At that point, a professional cleaning — or a fresh Windows install — is the right call.
Worried about malware on your computer?
Ray's Custom Computers provides professional virus removal for homes and businesses across Fayetteville, TN, Huntsville, AL, and McKinleyville, CA. We identify and fully remove infections, not just suppress them.
Published by Ray's Custom Computers — serving Fayetteville, TN, Huntsville, AL, and McKinleyville, CA since 1996. Questions? Contact us or call (931) 557-6104.