MSVCP100.dll was not found, what it means and how to fix it
MSVCP100.dll is the C++ standard library from Microsoft's Visual C++ 2010 package. Older games and tools built with Visual Studio 2010 need it to start, and the usual "was not found" message means the 2010 redistributable was never installed on this PC, or an uninstall or cleaner removed part of it.
The fix, in order
Install the Microsoft Visual C++ 2010 Redistributable Package from Microsoft, both the x64 and x86 versions. A 32-bit program needs the x86 package even on a 64-bit PC. Restart the PC when done.
Already installed? Repair it instead. Settings → Apps → each "Microsoft Visual C++ 2010 Redistributable" → Modify → Repair, for both the x86 and x64 entries.
Reinstall or verify the program itself. For Steam games: right-click → Properties → Installed Files → Verify integrity of game files. A damaged program folder can keep failing next to a healthy runtime.
Run Windows Update, then restart. Servicing updates sometimes replace runtime files, and a pending reboot can leave mismatched versions in place.
Do not download MSVCP100.dll by itself. DLL download sites offer single files of random versions, sometimes bundled with installers nobody asked for. The DLL belongs to a matched set installed by Microsoft's package, that package is the fix, and it is free.
If it still happens
Look inside the failing program's folder for its own copy of MSVCP100.dll. Some programs ship a private copy, and if that copy is older or damaged it overrides the healthy system one. Rename it (add .bak) so the program falls back to the system DLL, then relaunch. If only one program fails while others work, that program is damaged, and a clean reinstall is the remaining move.
Common questions
What is the difference between MSVCP100.dll and MSVCR100.dll? They are two halves of the same 2010 runtime. MSVCR100.dll is the C runtime, MSVCP100.dll is the C++ standard library. Both arrive in the same redistributable, so installing it fixes both errors at once.
Do I need both the x86 and x64 packages? Yes, on a 64-bit PC. 32-bit programs use the x86 set and 64-bit programs use the x64 set, and you often cannot tell which a program is until it fails.