well you could use FDISK.
go to add/remove programs in control panel and under the start up tab create a start up floppy.
next boot from the floppy. when you get the a:\ prompt type fdisk. there you can delete all existing partitions and then create a new partition that occupies the entire hard disk space. you'll have to format your hard disk again which means you are going to lose all your data .
have a look at these links for more precise info:
or you could use a pay software like partition magic