win32.comet.a is an adware Trojan — more of a persistent nuisance than a catastrophic threat. It does not steal banking credentials or hold files for ransom. However, it should be removed promptly because the ads it displays can sometimes lead to more dangerous malware (scareware, fake tech support scams). Modern Windows Defender with real-time protection and a good ad-blocker (uBlock Origin) is usually sufficient to prevent it.
The designation refers to a specific, older family of Trojan horses (or potentially unwanted applications) targeting the Microsoft Windows operating system.
