The Fantom ransomware program masquerades as a Windows update. A malicious website or an email can trick people into running the ransomware file named WindowsUpdate.exe. It runs from the AppData\Local\Temp folder and displays a fake Windows Update screen. Once it runs, it encrypts your files and demands a ransom, displaying the message shown here.
Unfortunately, the criminals spreading Fantom may not even respond if you try to pay the ransom. To be safe, use a cloud-based service like our Webroot Security to protect against known ransomware variants and a continuous cloud backup service like SecureMyFirm.