Welcome to Apple Support Communities
If you have a Mid 2010 MacBook, you can use Internet Recovery to reinstall OS X > http://support.apple.com/kb/HT4718
Start your MacBook holding Command, Option (Alt) and R keys. Then, open Disk Utility, choose your hard drive at the top of the sidebar, go to Erase tab and erase the disk. Then, close Disk Utility and reinstall OS X.
You will have to install Windows again after installing Mac OS X Lion. Follow these steps to reinstall Windows > http://manuals.info.apple.com/en_US/boot_camp_install-setup_10.7.pdf
If you have an old MacBook, use the Snow Leopard DVD to reinstall Mac OS X. Start your computer holding the C key while your computer is starting. Select the language, go to Utilities menu > Disk Utility, and follow the steps I have given you