Something went wrong when Boot Camp should have started the Windows installation. Happened to me once. Nothing to worry about. The first issue you should fix, is resetting your start-up drive (the disk partition your Mac wants to boot from; Boot Camp sets this to the Windows partition)
- Boot your Mac and press the option (ALT) kay as soon as your Mac has chimed
- All your bootable disks should appear. Choose your Mac OS X disk
- After booting in OS X, reset yout boot disk in >system utilities >Startup disk to your OS X drive
After that, go to the Boot Camp assistent and remove the Windows partition. You can now shut down (not restart) your Mac and turn it back on. After rebooting, you can go to the Boot Camp assistent again and restart making a Windows partition and install Windows after that. Make sure you've got an original install disk of Windows XP SP2 or Vista