Guys, you won't belive me, but I FOUND A WAY. After reading on InsanelyMac & Co, that it's not possible to add a GUID-OSx86 to a Windows-Bootloader, I got it working anyway. Short insctruction (I'll write a article on InfiniteMac later):

Go to your Leopard HDD /usr/standalone/i386/guid/ and copy the "boot0"-file to your Windows HDD root.
Then add following line to c:/boot.ini:

C:\boot0=Mac OS X Leopard

Greetings,
-crawle