No need to use a command prompt if you are planning to reformat your hard drive...

Just boot from the OS CD, then when you are done formatting (assuming you have only one partition), then you can put your desired partition size, before you install your OS..

If you have a previous partition, you have to delete this to make a clean partition after you reformat your hard drive..

Just read what is on the screen when you are installing your OS..
there is an option there Delete Partition, Create Partition..
You don't have to run command prompt and use fdisk to partition your hard drive because as what yo say, you will reformat your PC..

Anyways, if you have a second physical drive and that is the one you want to reformat and create a new partition..
then do this, just type in your Run Command "diskmgmt.msc" (assuming you are on windows XP)
without the quote.. It is a GUI for partitioning and formatting (managing) your hard disk...

hope this helps...