What OS are you trying to install? I formated a computer for 98 last night. If you're installing 2000/XP, this will probably be useless but oh well. Here's what I did:
Backup up all important data on the harddrive onto another store.
Inserted my bootdisk and restarted the computer.
Typed format c: to wipe the harddrive. Make sure your format the correct drive! To be safe, remove any other harddrives you might have that you do not want to format. Since it was going to use 98, I formatted as FAT32 each time I used this command.
Used fdisk to create two partions: a 5gb one for Windows and important system utilities only and left the rest for programs and data. (The advantage of this is if your computer messes up, usually you can just format the first partition to replace Windows and keep your current documents.)
Remember to set the partition with Windows as the active partition. Decide wether 5gb will be enough for your needs. 5gb is 5120mb exactly (5 * 1024). The capacity will show up as 4.99gb which is annoying but never mind. (When entering in sizes, make sure you enter the right unit. Eg don't enter 5 as in 5 gb when it's expecting the answer in megabytes, or you'll end up with a very small harddrive!)
After creating the first partion, it should ask how much you want in the extended partition. (This may be called extended logical drive or something similar, I can't remember exactly) Put in all of the remaining space. This is not a single partition but rather how much you want available for your remaining partitions. It is a common mistake to assume that it is a single partion, and enter in only a portion of the remaining space, and end up with wasted harddrive space. After you've created the extended partition, you can create 1 or more partitions within it.
When this is done, you will need to reformat your new partions again. eg format c:. If you already have an existing harddrive inserted and don't want to lose its data, make sure you unplug it before you do this to be safe. If you can't/don't want to, then one way to check is typing this:
x: where x is the hardrive letter, c, d etc
dir If you get an error message, the disk is not formated. If you don't get an error message, this disk is already formated and may contain data.
After the formating is complete, insert your OS disk and begin installing. Make sure you install on the correct partition.
Hope this helps!!!