To delete a Windows partition on a computer with a single internal disk:
1 Start up in Mac OS X.
2 Quit all open applications and log out any other users on your computer.
3 Open Boot Camp Assistant.
4 Select "Restore the startup disk to a single volume" and click Continue.


To delete a Windows partition on a computer with multiple internal disks:
1 Start up in Mac OS X.
2 Quit all open applications and log out any other users on your computer.
3 Open Boot Camp Assistant.
4 Select "Create or remove a Windows partition" and click Continue.
5 Select the disk with Windows on it.
6 Select "Restore to a single Mac OS partition" and click Continue.

*Copied from Boot Camp Setup Guide*

Hope dat helps..