A hard drive is like your room. It has stuff in it - the operating system, drivers, programs, and files. If you want to clean it out, you have a couple options. The easiest way is just to find your Windows installation disc, boot from it, select your hard drive/main partition, and format it using FAT or NTFS (depends on the version of Windows you use). This will completely erase everything on your hard drive and let you reinstall Windows.
Make sure you make data backups and have your Windows installation disc, program discs, and appropriate drivers for your hardware before you format and reinstall.
Alternatively, if you just want to wipe a disk, try something like DBAN (google it for more info).