OpenGL Header errors

I'm not sure if this is a widely-known problem or not but here goes:

When I include the Gl.h file in 4 or more files (Header files for classes) I start getting compilation errors, saying that the GL.h file has errors in it:

d:\program files\visual studio\vc98\include\gl\gl.h(1152) : error C2144: syntax error : missing ';' before type 'void'
d:\program files\visual studio\vc98\include\gl\gl.h(1152) : error C2501: 'WINGDIAPI' : missing storage-class or type specifiers
d:\program files\visual studio\vc98\include\gl\gl.h(1152) : fatal error C1004: unexpected end of file found
Error executing cl.exe.

I'm guessing that there is nothing wrong with the GL.h file, but why does the inclusion of the header file create these errors?

If you need more information, please ask.

Under Windows, you need to include windows.h before GL.h.

Thank you!