Either put the typedef for DWORD into a third #include file and
have both headers include that file ... or put the typedef before
the #include "header1" inside of header2. The former option is
probably the more desirable of the two. Chances are that you'll
have other "global" things like DWORD that you might want to use
everywhere ... so it'd behoove you to put all of these really
common things inside one header file.

--Paul