Hackers can steal passwords through malware (malicious software) that's been installed on your PC without your knowledge. For example, it might be accidentally downloaded with something you do want, like a new screen saver. It’s important to clear your PC of viruses or malware before you change your password.
Is your PC running Windows?
Great! Windows Defender is free antimalware software included with Windows 8.1 and Windows RT 8.1, and it updates 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, you should set it to regularly get updates and scan your PC.