Malware (viruses, trojans, and etc.) that finds its way onto a user’s computer can steal crypto keys and send them to an attacker. Malware may be used with another technique like phishing, where an attacker tricks a user into downloading a malicious file with a malware payload.
Use a heterogenous hardware and software system for storing your funds, including hardware wallets. This creates a more difficult environment for malware to operate in.