Last night I was trying to install Windows 7 on my new MacBook Air, I started it and the Windows installation began. I deleted 80GB HD (preinstalled Mac OS) also deleted, but I could install windows completely. Now my MacBook is not starting or booting from external USB DVD rom, please help me out, not even booting from the original Mac CD which came with PC.
If you have one of the brand new Macbook Airs (11" or 13") it comes with a Restore USB drive. You will have to reinstall a barebones OS X install with BootCamp Drivers to get your Win7 machine up and running. You may have to do a complete reformat in Disk Utility while booting off the USB drive and start your Win7 install over again. Apple laptops are made to run OS X primarily and Win7 secondarily. You can run Win7 all you want and only boot into OS X once to get the computer set up but you need to keep that 8GB partition for OS X on your computer.
As Faiz said, you will boot to the USB Recovery by plugging it in and holding *Option* and choosing the USB Recovery as the boot source. Installing OS X is as easy as Win7 so just follow the prompts and you'll be good.