Beware the signs of a potentially malware-infested PC: slower-than-usual performance, the recent occurrence of lots of pop-ups, and other weird issues. It’s possible your system has been infected by a virus, spyware, or other nefarious entity—even if you have an antivirus program installed. Yes, out-of-the-ordinary behavior is sometimes the result of hardware issues, but it’s best to first rule out malware if your PC is acting up. Here’s a step-by-step guide for taking action.

Step 1: Enter Safe Mode

Before you do anything, you need to disconnect your PC from the internet, and don’t use it until you’re ready to clean your PC. This can help prevent the malware from spreading and/or leaking your private data.

If you think your PC may have a malware infection, boot your PC into Microsoft’s Safe Mode. In this mode, only the minimum required programs and services are loaded. If any malware is set to load automatically when Windows starts, entering in this mode may prevent it from doing so. This is important because it can make removing the nefarious files easier since they’re not actually running or active.

You may find that your PC runs noticeably faster in Safe Mode. This could be a sign that your system has a malware infection, or it could mean that you have a lot of legitimate programs that normally start up alongside Windows. If your PC is outfitted with a solid-state drive it’s probably fast either way.

Step 2: Delete temporary files

Now that you’re in Safe Mode, you’ll want to run a virus scan. But before you do that, delete your temporary files. Doing this may speed up the virus scanning, free up disk space, and even get rid of some malware. To use the Disk Cleanup utility included with Windows 10 just type Disk Cleanup in the search bar or after pressing the Start button and select the tool that appears named Disk Cleanup.

Step 3: Download malware scanners

Now you’re ready to have a malware scanner do its work—and fortunately, running a scanner is enough to remove most standard infections. If you already had an antivirus program active on your computer, you should use a different scanner for this malware check, since your current antivirus software may not have detected the malware. Remember, no antivirus program can detect 100 percent of the millions of malware types and variants.

There are two types of antivirus programs. You’re probably more familiar with real-time antivirus programs, which run in the background and constantly watch for malware. (Another option is an on-demand scanner, which searches for malware infections when you open the program manually and run a scan. You should have only one real-time antivirus program installed at a time, but you can have many on-demand scanners installed to run scans with multiple programs, thereby ensuring that if one program misses something a different one might find it.

Step 4: Fix your web browser

Malware infections can damage Windows system files and other settings. One common malware trait is to modify your web browser’s homepage to reinfect the PC, display advertisements, prevent browsing, and generally annoy you.

Before launching your web browser, check your homepage and connection settings. For Internet Explorer right-click the Windows 10 Start button and select Control Panel, then Internet Options. Find the Home Page settings in the General tab, and verify that it’s not some site you know nothing about. For Chrome, Firefox, or Edge, simply go to the setttings window of your browser to check your homepage setting.