You can use Regsvr32.exe to register 32bit dll on Windows 7 or Windows 2008 64bit.

Please read instruction below:

Extract dll file, copy to a folder C:\website

Open command prompt window  and run command

cd C:\Windows\syswow64
Regsvr32 C:\website\your-custom-dll.dll

You should receive response confirming dll is registered successfully.

There is another version of Regsvr32 in C:\Windows\System32\ which is the default 64bit version.