Kid installed XP over OSX, help!
The first thing to do is to eject your CD/DVD and then power down the Mac and then disconnect any drives and accessories other than your keyboard and monitor.

Then boot the Mac and as soon as you hear the fans start or the chime - hold only the option key down.

This should boot to a grey screen that lists any bootable OS that are connected.

At this point, see if you can see a Mac partition to boot to. Worst case, if you only see windows, try installing the DVD/CD and boot to disk utility. Don't erase things, just use it to see the partition scheme and report back what it shows. A cell phone picture of the window would be a good start to see what damage if any has been done.

Don't agree to any prompts to reformat any drives for now. Also, consider paying for AppleCare to walk you through this reinstall or visiting a local genius bar. Sometimes having a thorough back and forth dialog is far better than posting updates to the internet which can take days or weeks to cover what 15 minutes of troubleshooting from a trained pro can accomplish.

The best case for you is that the system forgot that it has a Mac partition ready to boot and you can re-"bless" that partition and be back in business in 10 minutes. Worst case, you'll end up doing an erase/install per the guide here from Apple:


http://support.apple.com/en-us/HT3910


On second thought, that isn't the worst case. Worst case is you have a hardware error and have to triage and fix that before you can follow the steps that normal work to reinstall the OS of your choice. From the details, you certainly have backup media and possibly a data backup so even getting another computer to load your data is an option. You might be able to find a very inexpensive Mac Mini or such to keep running the OS of your choice if you don't want or need to move up to newer hardware.