Some argue that Metal Slug Complete PC (New) is abandonware because SNK has discontinued physical PC sales. Sites like Archive.org house ISO copies of the original discs. While downloading these ISOs is a legal gray area, is the only way to bypass the serial requirement without buying a used key.
If you absolutely want an offline all-in-one “Complete” experience, download a pre-configured MAME package from a trusted source (after legally acquiring the ROMs) — but again, no serial number will ever be required or found.
Check the front cover, back cover, or the very last page.
: Older physical "Complete" versions for PC may contain a serial sticker on the back of the manual or inside the jewel case. If you have lost this code, you may need to contact the original publisher, though many of these retail versions are now handled via digital storefronts for better compatibility with newer OS. Included Titles in the "Complete" PC Version Metal Slug Complete PC
There is no single “complete” package on Steam. Instead, SNK sells titles individually (Metal Slug 1, 2, X, 3, 4, 5). For Metal Slug 6 and 7 (renamed XX), you buy Metal Slug XX separately.