If you don't know how to get back to MAC OS X from Windows XP, this is how. Look down in the right hand corner where the clock is, you see all the icons beside the clock? Look for a grey diamond, it should tell you it is BootCamp when you put the mouse cursor over it. Click on that and it will give you the option to restart in MAC OS X. From there do what k3v said above me. Now, I just thought of something else. Did you install BootCamp in Windows XP? If you didn't, you need to put OS X Install Disk 1 DVD/CD in and install it, otherwise there are other things that won't work correctly (Apple drivers neded by windows to use your Apple hardware correctly). If you have already done that just do what I said above and you will be good.
Hope that helps