https://support.micr...orrupted-system
For antivirus
avast! Free Anti-Virus (free)
BitDefender Anti-Virus (free)
Microsoft Security Essentials (free)
Sophos Home (free)
Emsisoft Internet Security (paid)
ESET NOD33 Anti-Virus (paid)
Kaspersky Anti-Virus (paid)
Panda Anti-Virus Pro (paid)
Trend Micro Antivirus+ Security (paid)
Webroot Anti-Virus (paid)
For more suggestions see Choosing an Anti-Virus Program which includes more details why Windows Defender Antivirus with Exploit Guard designed to protect against ransomware attacks is a very good choice.