Make sure you're on the desktop of your Windows installation.
Press WIN+R to open the Run prompt, then type DISKPART and press Enter.
Now we're going to use DISKPART to create a HFS+ compatible Mac partition that we can boot off.
Follow these next set of commands:
DISKPART> list disk
Will show you a list of selectable disks
DISKPART> select disk n
[where n = the disk number]
This will specify which disk to create a new partition on, assuming you have already used the XP Install Disc, Partition Magic or Windows Disk Management to set aside some unformatted space. If not, and the drive you want to install Mac OS X on is empty, just follow the instructions immediately below for creating a partition.
DISKPART> create partition primary size=n id=af
[where n = the size in MB of the partition, however if no size is specified then the remaining space of that drive is used]
[where af = hfs+ compatible]
Will proceed to create a bootable primary partition for the Mac
For example: create partition primary size=100000 id=af
This will create a 100GB partition for Mac OS X.
DISKPART> list partition
This will show you the partitions on your selected disk
DISKPART> select partition n
[where n = the newly created OSX partition]
This will specify the partition
DISKPART> active
Will make your OS X partition the first-boot active partition
DISKPART> Exit
Leaves DiskPart...
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