about 4 years ago - Shurenai - Direct link
If a game is on steam it will typically have been vetted a number of times, scanned multiple times, etc, before ever being made availalbe for a consumer to download. Moreover, The forums would be awash with "THIS GAME IS MALWARE!" posts; and steam would have torn it down ages ago.

Windows is hilariously oversensitive somtimes when it comes to malware; many security programs are. Most often they're twitchy over 'New' files, because anything new hasn't been tested by the company that owns the security program and thus anything new is, by default, Malware in the programs eyes. These false positives typically get reported as something like "Generic-Malware-Blahblah-Xyz"; which essentially translates to "We don't know what this is, so we're acting against it just in case."

It's not malware. You can take my word for it, Or not; It's your choice and your computer.