How do I register a DLL file on Windows 7 64-bit?

If the DLL is 32 bit:

Copy the DLL to C:\Windows\SysWoW64\
In an elevated command prompt: %windir%\SysWoW64\regsvr32.exe %windir%\SysWoW64\namedll.dll

if the DLL is 64 bit:

Copy the DLL to C:\Windows\System32\
In an elevated command prompt: %windir%\System32\regsvr32.exe %windir%\System32\namedll.dll

I know it seems the wrong way round, but that's the way it works. See:

http://support.microsoft.com/kb/249873
Quote: "Note On a 64-bit version of a Windows operating system, there are two versions of the Regsv32.exe file:
The 64-bit version is %systemroot%\System32\regsvr32.exe.
The 32-bit version is %systemroot%\SysWoW64\regsvr32.exe.
"

Well, you don't specify if it's a 32 or 64 bit dll and you don't include the error message, but I'll guess that it's the same issue as described in this KB article: Error Message When You Run Regsvr32.exe on 64-Bit Windows

Quote from that article:

This behavior occurs because the Regsvr32.exe file in the System32 folder is a 64-bit version. When you run Regsvr32 to register a DLL, you are using the 64-bit version by default.

Solution from that article:

To resolve this issue, run Regsvr32.exe from the %SystemRoot%\Syswow64 folder. For example, type the following commands to register the DLL: cd \windows\syswow64 regsvr32 c:\filename.dll

Type regsvr32 name.dll into the Command Prompt (executed in elevated mode!) and press "Enter." Note that name.dll should be replaced with the name of the DLL that you want to register. For example, if you want to register the iexplore.dll, type regsvr32 iexplore.dll.