What happens when you restart the computer and hold down the option key? If you see Macintosh HD & Windows, select Macintosh HD, then it will boot into OS X. Go to System Preferences and change your startup disk to your Macintosh HD. Or, if you're in Windows, go to your Control Panel and select Startup Disk, then select Macintosh HD.
If you hold down Option when you reboot the computer and don't see your Macintosh HD, you formatted the wrong partition when you installed Windows and erased OS X. If that's the case, you'll need to boot from your startup cd (insert the cd, reboot the computer, and hold down the C key as it's restarting, go to Disk Utility, format your harddrive as HFS+ (Journaled), and reinstall OS X.