Many computers come with a system recovery CD which contains everything you need to get your computer back to its original state. If you have one of these CDs, place it into your drive and boot your computer. It will lead you through the process.
If you don't have a recovery CD, then you should use the Windows CD. When you boot off the CD, you'll probably be given a choice of fixing Windows or reinstalling. Choose a reinstall, and if you are given the option of reformatting the C: partition, you should choose that option to ensure that everything on your drive is first erased.
If these instructions aren't sufficient, please post more details about your system, and let us know what system CDs you have. There are sometimes better ways of fixing a system than erasing the hard drive and starting over from scratch, so if you tell us why you're doing this, someone may be able to come up with an easier solution (erasing your hard drive means you'lll need to reinstall all programs, and also restore any data, such as letters, documents, spreadsheets, databases, pictures, etc. that you may have created -- assuming that you've copied them someplace safe).
If you don't have a recovery CD, then you should use the Windows CD. When you boot off the CD, you'll probably be given a choice of fixing Windows or reinstalling. Choose a reinstall, and if you are given the option of reformatting the C: partition, you should choose that option to ensure that everything on your drive is first erased.
If these instructions aren't sufficient, please post more details about your system, and let us know what system CDs you have. There are sometimes better ways of fixing a system than erasing the hard drive and starting over from scratch, so if you tell us why you're doing this, someone may be able to come up with an easier solution (erasing your hard drive means you'lll need to reinstall all programs, and also restore any data, such as letters, documents, spreadsheets, databases, pictures, etc. that you may have created -- assuming that you've copied them someplace safe).
