If your intention is to reinstall your operating system, by formatting the hard drive
it is best done with the installation CD , when you begin installing, it asks you to format your hard drive.
Note that if the hard drive has partitions, only one partition is formatted (the one you select)
If you want to format harddrive partitions which do not contain OS on them you can do that within the operating system.
Any computers connected to the network should not be affected, unless you have permission to perform this operation on them as well. In that case just make sure not to select them.