How do I delete the windows operating system?

Use "Darik's Boot and Nuke" to create a bootable CD you can use to wipe the drive.

Go here and follow the insructions that apply to you.

http://www.dban.org/

EMD

We use a program called PC Scrub to completely erase a hard drive prior to disposal. If your selling your old PC's, make sure you do a complete erasing of you hard drive. Formatting does not completely delete your data. That could risk personal information being retrieved from your computer.

Many people here work in roles where they need to rid any hope of recovering sensitive company data from a hard drive. Miltary grade wipes and complete dsetruction of cylinders etc seem to have IT staff all excited about trashign drives.

For the rest of the working world, a reformat and fresh install (if wante) works just fine. Most people, especially those buying used PC's have little or no interest in going into depth with data recovery or paying a recovery centre to do so and wouldn't have use for most recovered data, if they incurred the expense, anyway.

Format it, sell it and you're off to the races with VERY little effort.