Not only will you need to reformat the drive, that's actually going to be the second step. First step is you need to repartition the drive, but you should be able to do that during the installation process depending on your OS.
Assuming Win7:
Put the installation in the drive (or plug in the media if installing via USB stick).
Boot the computer.
Get into the bios before any make sure the removable drive/dvd/etc are set to bootable.
Reboot.
Win7 Installation should start.
There's a point where it will ask which disk/partition you're installing to.
Click 'advanced'. This will open up more options.
Click each partition and hit the delete icon until there is only raw space left.
Select that empty raw partion and hit next (or install, can't remember exactly which it is).
If you're using a different OS (Linux/Win8) there's a different process, but the basics are roughly the same in that you're going to need to remove all partitions out, then install.