diff --git a/reactos/include/ddk/pnptypes.h b/reactos/include/ddk/pnptypes.h
index 61529f4b104..2ba54539fc6 100644
--- a/reactos/include/ddk/pnptypes.h
+++ b/reactos/include/ddk/pnptypes.h
@@ -5,8 +5,8 @@ struct _DEVICE_OBJECT;
 struct _FILE_OBJECT;
 
 // windows.h may be included before
-#ifndef GUID_DEFINED
-#define GUID_DEFINED
+#ifndef _GUID_DEFINED
+#define _GUID_DEFINED
 
 typedef struct _GUID {
   ULONG Data1;
diff --git a/reactos/include/net/ndis.h b/reactos/include/net/ndis.h
index d1f8b8bbaa2..a0184571e6a 100644
--- a/reactos/include/net/ndis.h
+++ b/reactos/include/net/ndis.h
@@ -197,8 +197,8 @@ typedef CONST CHAR *PCSTR;
 
 #if defined(NDIS_MINIPORT_DRIVER) || !defined(_MSC_VER)
 
-#ifndef GUID_DEFINED
-#define GUID_DEFINED
+#ifndef _GUID_DEFINED
+#define _GUID_DEFINED
 
 typedef struct _GUID {
     ULONG   Data1;
@@ -207,7 +207,7 @@ typedef struct _GUID {
     UCHAR   Data4[8];
 } GUID;
 
-#endif /* GUID_DEFINED */
+#endif /* _GUID_DEFINED */
 
 #endif /* NDIS_MINIPORT_DRIVER || _MSC_VER */
 
diff --git a/reactos/include/ole32/guiddef.h b/reactos/include/ole32/guiddef.h
index 5b41edbfa28..9e10326bf63 100644
--- a/reactos/include/ole32/guiddef.h
+++ b/reactos/include/ole32/guiddef.h
@@ -43,8 +43,8 @@ Cambridge, MA 02139, USA.
 
 
 //	guid definition
-#ifndef GUID_DEFINED
-	#define GUID_DEFINED
+#ifndef _GUID_DEFINED
+	#define _GUID_DEFINED
 	typedef struct _GUID {
 		unsigned long  Data1;
 		unsigned short Data2;
diff --git a/reactos/include/ole32/rpc.h b/reactos/include/ole32/rpc.h
index fe2e48f4628..5cec503db19 100644
--- a/reactos/include/ole32/rpc.h
+++ b/reactos/include/ole32/rpc.h
@@ -18,8 +18,8 @@ typedef long RPC_STATUS;
 
 typedef void* I_RPC_HANDLE;
 
-#ifndef GUID_DEFINED
-#define GUID_DEFINED
+#ifndef _GUID_DEFINED
+#define _GUID_DEFINED
 typedef struct _GUID
 {
     DWORD Data1;
diff --git a/reactos/include/structs.h b/reactos/include/structs.h
index a8decaabcf6..7f7587cc90e 100644
--- a/reactos/include/structs.h
+++ b/reactos/include/structs.h
@@ -257,10 +257,13 @@ typedef struct {
   DWORD        bV4GammaBlue;
 } BITMAPV4HEADER,   *LPBITMAPV4HEADER, *PBITMAPV4HEADER;
 
+#ifndef _BLOB_DEFINED
+#define _BLOB_DEFINED
 typedef struct _BLOB {
   ULONG   cbSize;
   BYTE    *pBlobData;
 } BLOB;
+#endif
 
 typedef struct _SHITEMID {
   USHORT cb;
@@ -2794,8 +2797,8 @@ typedef struct _SERVICE_ADDRESSES {
   SERVICE_ADDRESS   Addresses[1];
 } SERVICE_ADDRESSES, *LPSERVICE_ADDRESSES;
 
-#ifndef GUID_DEFINED
-#define GUID_DEFINED
+#ifndef _GUID_DEFINED
+#define _GUID_DEFINED
 typedef struct _GUID
 {
     unsigned long  Data1;