if your disk is a system disk, in this way:

find an installation CD of OS, and pretend to install the system, at the step where to install the OS:

delete the old system parittion, then re-create the new partition and format.

Using the installation procedure to format a disk is very slow, if your disk is big, such as 160 GB and above, suggest to create a small partition to format, the rest could be formated after the system is up.

Or just delete the old system partition, then quit the installation procedure, and put this disk in any other computer to format to save time.