I was wondering how cool it be if it is possible for a desktop computer to be capable to have Microsofts Windows 7 and Apple IOS snow leopard or higher.Just throwing this idea out there but it might have been done already. 
You can do this but I suggest you do a little research because it matters which OS is installed first.
I have a dual boot Linux/ Windows XP Dell inspiron 1525. I found the info I needed on the internet. I did this because I was starting my Linux Admin Class at ITT Tech and was given the same warning about researching it.
The main issue is the fact that windows always rewrites the MBR. It may be easier to just use VirtualBox which is a free emulator, which will effectivly allow you to run an OS inside of another OS. It would most likely be easiest to make 3 partitions and use Linux Windows and Mac installing Linux last as it will write the MBR with GRUB which can autodetect the other OSs.
The main issue is the fact that windows always rewrites the MBR. It may be easier to just use VirtualBox which is a free emulator, which will effectivly allow you to run an OS inside of another OS. It would most likely be easiest to make 3 partitions and use Linux Windows and Mac installing Linux last as it will write the MBR with GRUB which can autodetect the other OSs.