To return your HD to the factory settings do a full system recovery using the recovery disks (CD or DVDs) you made when you first had the computer. This will return the whole computer to its as-shipped state: you will lose all your data and any software that you have installed. This will retain/re-install the original partition structure on your hard disk. To do this process you may also use the recovery partition on the hard disk.
To completely reformat the hard disk, in which case you will lose everything on it
press the start button at the bottom left of screen,
R-click on computer, and select manage. The computer management window will open. From the left pane click on disk management under 'storage'. A new pane will open, with information on the different disks in your computer. Select the disk on which the partition to be formatted is located. R-click on the partition to be formatted, and choose format, and ten work through the format wizard. (see http://windows.microsoft.com/en-us/windows/create-format-hard-disk-partition#create-format-hard-disk-partition=windows-7
When you R-click on the partition other options will appear will appear as well as format, choose the appropriate option to create a new partition, merge, etc, By successively working your working your way through the partitions you may format/merge some while retaining others.