Using a legitimate serial number or buying the game through official channels offers several advantages over unofficial, "cracked" versions:

Losing a activation key for a decade-old PC game is incredibly common. Unfortunately, standard customer support routes for these legacy physical editions are no longer active, as SNK and the original PC port distributors have shifted focus away from old DRM systems. The Danger of Online "Serial Key Generators"

If you prefer DRM-free gaming, GOG offers several classic Metal Slug titles.

The Metal Slug Collection (often referred to as Metal Slug Anthology or a bundle of the mainline games) brings together the classic arcade titles in one package for modern Windows PCs. These games typically include: (1996) Metal Slug 2 (1998) Metal Slug X (1999) Metal Slug 3 (2000) Metal Slug 4 (2002) Metal Slug 5 (2003) Metal Slug 6 (2006) Metal Slug 7 (2008)

Right-click the executable, go to Properties , open the Compatibility tab, and set it to run in compatibility mode for Windows XP (Service Pack 3) or Windows 7 .

The Ultimate Guide to Metal Slug Collection PC Serial Numbers, Setup, and Troubleshooting

Check the box for and select Windows XP (Service Pack 3) or Windows 7 . Check Run this program as an administrator and click Apply. 2. Check for Official Patches

Compare the of the anthology vs individual releases . Give you a quick guide on how to set up a USB controller .