Windows NT contains some useful functions for updating resources in executables. Those are, however, not implemented on Windows 9x, so I had to write my own library for doing it. My library works in exactly the same manner as the BeginUpdateResource(), UpdateResource() and EndUpdateResource() API functions, just prepend an underscore to their names. It also provides a new function, _GetRsrcData(), which is used to get the resource data for a resource in a file loaded with _BeginUpdateResource().
The code works, at least on my system, but it has one major drawback: it can't move a section in memory. This means that if the .rsrc section would have to be expanded past its padding, _EndUpdateResource() will return ERROR_CANT_MOVE_SECTION unless .rsrc is the last section (both in the file and in VM space). This is a problem because VC6 puts the .reloc section (relocation info, used in DLLs and debug EXEs) after .rsrc, so you can't use the functions very well on those file types.