You might have heard of img/lnk/other files infecting boxes just when Explorer tries to access it.
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Well first of all I'd suggest you to stop seeing those keygen files, as they are hacked piece of softwares (mostly/sometimes) accompanied with some other non-harmful looking image/nfo/lnk files which in full capacity of the hackers/crackers intentions could be crafted especially to infect your machine itself. Technically - preventing from running under debugger, combined with encrypted code in file (with in-memory decryption, possible - multi-layer for some sensitive parts), chains of memory manipulations, online verification of keys in external storage (see previous methods, which are applicable separately to checker also).