The legitimate isarcextract.dll is not a virus. However, malware sometimes uses the same name. Always check the digital signature of the file. Right-click the DLL > Properties > Digital Signatures. A valid signature should indicate the publisher as "Jordan Russell" or related to Inno Setup.
Your best bet for a safe copy is to re-download the original installer from a trusted source (like the game's official website or a reputable platform like Steam or GOG). A fresh download will contain all the necessary, unaltered DLL files. install isarcextract.dll