If you are using bootcamp, use the bootcamp assistant to delete the partition. If you want to reinstall in the same size partition, there's no need to delete -- just install over the old version. If you want to install in a different size partition, delete the old one and install again selecting a different size. (Or search the forum for ways to alter the partition sizes). Deleting the partition will recover HD space.
If you're using Parallels, there's a delete VM choice on the VM menu. To reinstall, just create a new VM. The default install will create an expending virtual disk that takes up only as much space as it needs for files you put on it. You can expand it, or create another virtual disk and attach it to your VM if you need more space.