Have you looked at the following Appication note: <a href=http://zone.ni.com/devzone/conceptd.nsf/webmain/5CF9A9FFD774028586256869005FF2ED/$File/AN087.pdf>Writing Win32 Dynamic Link Libraries (DLLs) and Calling Them from LabVIEW</a>.
There are many other resources dealing with <a href=http://zone.ni.com/devzone/devzone.nsf/webcategories/E2A99E7E10D5725D862567AC004F0A53>Integrating DLLs</a>
