Do you mean remove the current data and start again? Simplest way is to reformat. Get your Windows disk, pop in the disk, boot and let it run the setup. When asked, delete current partitions or reformat the current partition. You could also use your hard drive manufacturers low-level format utility to write zeros to the drive, but really that is excessive just to "start over". Reformat and you will be all set. (You can not reformat a system drive from within Windows.)
\Dan