While the term "free download" is often associated with unofficial sites, there are several legitimate ways to get the full version of Metal Slug X on your computer:

This version introduced the "Iron Lizard" (remote-controlled missiles) and the "Drop Shot" (bouncing landmines). Save 80% on METAL SLUG X on Steam

You can find the game as part of the Metal Slug Anthology or via the ACA NEOGEO series on various stores, which are officially licensed ports of the arcade original. Alternative "Free" Options:

System Requirements * OS *: Windows Vista, 7, 8. * Processor: Pentium 4 2.4Ghz. * Memory: 1 GB RAM. * Graphics: Intel HD Graphics. Metal Slug X - Download

Face off against giant mechanical tanks, alien saucers, and the ever-persistent General Morden.

Sites like Uptodown host fan-made "Fanthology" versions or remakes that are often free but may vary in quality and accuracy.