Hackers can steal passwords through malware (malicious software) that's been installed on your computer 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 computer of viruses or malware before you change your password.
Is your computer running Windows?Great! Windows Defender is free anti-malware 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 anti-virus program, you should set it to regularly get updates and scan your PC.