Description from Microsoft.

Windows Defender is a free program that helps you stay productive by protecting your computer against pop-ups, slow performance and security threats caused by spyware and other potentially unwanted software. Windows Defender no longer supports Windows 2000 as mainstream support ended in June 2005.

Download Windows Defender Here

Follow up:

After you donwload Windows Defender, run the program and install. You will be prompted to download updates and run program.

Now the scan will run and find any infections.