You've come to this page because you've said something to the effect that you intend to put, or that you are putting, or that it is a good idea to put, one of your own DLLs in the %SystemRoot%\System32 directory.
This is the Frequently Given Answer to such statements.
Don't put your DLL into the System32 directory.
The fact that every man and his dog arrogantly thinks that his DLL is important enough to be placed in the operating system directory is part of the reason why Windows is in the mess that it is in now.
The advantage of this is that however many of those applications you run, they will all share the one DLL. If things work right, they may even end up all sharing a single set of code and (constant) data pages for your DLL.
The advantage of doing this is that you don't have version control nightmares when one application requires one version of the DLL and another application requires a different version. Having each application have its own copy of the DLL eliminates this problem. Two or more processes in Win32 can load completely different DLLs, even with the same basename, without conflict. Also, your DLL will be installed and uninstalled along with the application as appropriate, rather than being an ugly "dropping" that gets left behind if the application is uninstalled, as is sadly the case with so much badly thought out Windows software.
And always remember not to hard-code names such as "\Program Files" or "\Program Files\Common Files", and especially not to hard-code drive letters. Use the various available helper DLLs that export functions to look up these names. Or if that isn't possible, try to use the values of the %PROGRAMFILES% and %COMMONPROGRAMFILES% environment variables.