I'm not going to ask why you have windows XP and no OS X on your Mac.
You need to purchase OS X.
If your Mac meets the min requirements for Snow Leopard, you can get it for around $25-30 from Apple, Apple resellers, Amazon, etc.
You could also ring Apple and have them send you out the original install discs for a little less than that

Snow Leopard requirements:
Mac computer with an Intel processor
1GB of memory
5GB of available disk space
DVD drive for installation
Clean install is what you will need to do.
-Boot from the install disc holding C while booting until you see the spinning gear.
-Select language and click continue.
-Select utilities>disk utility in the menu bar (this takes a few seconds to appear).
-Select the HD at the top of the list on the left.
-Click erase tab.
-Format Mac Os Extended (Journaled).
-Name Macintosh HD.
-Click erase.
-Quit disk utility menu, and continue with installation.
This is what you'll need to