There is no msvcp100.dll on the Windows XP SP3 installation CD and Windows File Protection (and therefore SFC) does not know or care about the file.
If you perform a clean install of XP SP3 you will not find msvcp100.dll on your system.
How could it get installed on a system if it is not even on the installation CD? Magic? Of course not. It got installed from something else.
If you find it on your system it is there because something else you installed put it there. The file gets installed when you install other programs that need it of which there are many. If there is some problem because the file suddenly appears missing,
it could be because or over zealous "cleaning", it may have been quarantined (effectively deleted) by some antivirus/antimalware application or the uninstallation of some other program. Files don't go missing by themselves.
Since my XP install is 10+ years old and I have installed/uninstalled thousands of applications in that decade I have no idea when that file got installed onto my system or what program installed it but it is there for some reason and I really don't care why.
It is there, I don't know or care how it got there but if somebody thinks having it is going to solve a problem, feel free to download and apply it and hope it is the right one they need.