Boot into the partition you want to keep. You're going to want to edit the bootloader to only have the OS install you want so hit - R and type in 'msconfig' there.
You need to go to the boot tab and remove any OSes you don't want
Be very careful here since if you remove the wrong one you may render the system unbootable. It should be logical not to since it shows what's the current OS, and what's the default OS :). Set the current OS to default if it isn't, select the other one, and delete it. This should keep it from booting.
You can then format the partition, and use a third party disk management tool if you want to add it to the main partition.