After installation, you may still need the CD to play. Download a (from reputable abandonware sites) or mount the original ISO using Daemon Tools with "RMPS emulation" enabled.
You bought a legitimate physical copy of the game years ago. You still have the disc, but the original packaging—where the serial number was printed on the back of the manual or on the jewel case—has been lost or thrown away. You own the game, but you cannot install it. Numero De Serie De Metal Slug Collection Pc
In the mid-2000s, digital distribution platforms like Steam did not dominate the market yet. To combat piracy, physical CD/DVD releases used or Serial Numbers. The Metal Slug Collection PC uses SafeDisc or SecuROM (depending on the regional version). During installation, the screen freezes on a dialogue box asking for the "Manual Key" or "Serial Number." After installation, you may still need the CD to play