Compile dll in both 32 bits and 64 bits

We have a C/C++ library source code. After we compile it into dll, we have problem to load it in both Windows 7 64 bits and Windows XP 32 bits. So we need someone to recompile the code into both 32 bits and 64 bits dll, write a simple app to load the correct dll depend on the system and also write a simple installer, so we can install on our computers to verify. It should be a simple job for experienced coder.