Many hackers steal passwords through malware (malicious software) that's been installed on your PC without your knowledge (for example, when you download a new screen saver or toolbar). It’s important to clear your PC of viruses or malware before you change your password. That way, the hackers won’t get your new password.
If you’re using Windows 8.1 or Windows RT 8.1, Windows Defender is free antimalware software included with Windows, and you can update it automatically through Windows Update. If you're running an earlier version of Windows, you can download and install Microsoft Security Essentials for free.
After you install an antivirus program, set up the program to get updates and scan your PC on a regular basis.