If you search for you will find dozens of shady sites. Do not click them. Instead, wait for a Steam sale or buy the DRM-free version on GOG.
Disclaimer: This blog does not host or link to pirated software. We strongly encourage supporting game developers by purchasing legal copies.
If you are feeling a wave of nostalgia and want to replay it—or try it for the first time—you might be tempted to search for
mini-ninjas-free-pc-guide