Registering and Registering a .dll from code

I am trying to register a dll from my code(Windows 7, 64-bit Application). The code snippet as below.

hinst = ShellExecute(NULL, _T("open"), L"regsvr32.exe", str, NULL, SW_SHOWNORMAL);
str -> fully qualified path of the file like  C:\\XXXX\\XXXX\\XXXX.dll"

The ShellExecute returns the following:

"0x0000002a" and fails to register the file

What could be the issue?