stdrt.exe is the compiler of Multi Media Fusion 2.0.
Knytt Stories is made using MMF2, and thus stdrt.exe is the internal name of the file. If someone made a virus in MMF2, and virus scanners pick this up, they'll basically say that everything made with MMF2 is a virus, which isn't the case obviously.
Then, sometimes the compression algoritm that is used to create the stdrt.exe is identified as a possible malware which is a false positive.
In these cases, always scan with a 2nd scanner just to be sure, because chances are 99% that you have a false positive, rather than a real virus.