: Third-party DLLs can be infected with malware, viruses, or designed for "DLL Hijacking," allowing attackers to gain control over your system.
Type appwiz.cpl and press to open Programs and Features.
If re-adding the reference doesn't work, repair your Office installation:
: The library is often contained within Excel.exe or MSO.DLL located in the OFFICE12 folder .
The best way to fix missing Excel DLLs is to run a repair on your installation. Go to Control Panel > Programs and Features , find your Microsoft Office installation, right-click, and select "Change" then "Repair."
It is contained within a file named EXCEL.EXE or specialized Primary Interop Assemblies (PIAs) like Microsoft.Office.Interop.Excel.dll .