Malware means software designed to function in ways that mistreat or harm the user. (This does not include accidental errors.) This page explains how Microsoft software is malware.
Malware and nonfree software are two different issues. The difference between free software and nonfree software is in whether the users have control of the program or vice versa. It's not directly a question of what the program does when it runs. However, in practice nonfree software is often malware, because the developer's awareness that the users would be powerless to fix any malicious functionalities tempts the developer to impose some.