Programs and applications often require other files to be present on your computer, but sometimes this isn't the case. DLL stands for Dynamic Link Library and these files are used by many programs including games.
MSVCR100.DLL is a Microsoft Visual C++ file and is required by applications which were made with Visual C++. However, while downloading this file and copying it to the Windows\System folder should fix the error for most people, it might need to be copied to the game or program's folder, so you'll need to know which application threw up the error.
How to fix msvcr100.dll is missing error
Step 1: Head to the DLL-Files website and simply scroll down until you see the download links (you don't have to pay - despite appearances). This is a trusted site, but you can also find the file on other websites, such as Fix4dll. When presented with a list of various versions, choose the one appropriate to your system: 32-bit or 64-bit. Go for the latest version.
Step 2: Unzip the file you just downloaded - double-clicking on the file (which is likely to be in your Downloads folder unless you specified a different download directory) will open it using Windows' built-in support for zip files.
Step 3: Extract the file msvcr100.dll to you the C:\Windows\System32 folder - this assumes you have used the default hard drive letter for Windows. If your system is running 64-bit Windows, also copy the file to C:\Windows\SysWOW64. (If that folder doesn't exist, you probably don't have 64-bit Windows.)
Step 4: Launch the application or game that caused the error, and it should now run - or complain that another DLL file is missing. If it still asks for the same file, try rebooting your computer and if that doesn't fix it, then copy msvcr100.dll to the application or game's installation folder in C:\Program Files or C:\Program Files (x86).