Sounds like a plan...but a few questions need to be answered first:
Do you have more than one partition on your harddrive?
If you do have more than 1, do you wish to reformat *all* of them, or just one?
Which Linux are you running?
If your machine is running Linux *only* (i.e. not dual-booting with Windows), then it is likely that you have more than one partition: At least the following:
1 x Swap partition (Lin uses this as 'fake' RAM)
1 x Linux native (for your programs, etc).
If you're running Mandrake then there should have been a program called DiskDrake installed. This is intuitive, easy to use and VERY DANGEROUS if you don't know what you're doing! Run it to find out how many partitions you've got (you _can_ reformat them from here aswell).
If you've got multiple partitions and you only want to re-format some of them, without destroying your Linux system, use DiskDrake, select the relevant partitions & format. **ALL DATA WILL BE LOST**
If you wanted to completely re-do the whole install thing, then do it. Get your installation CD, boot from it and start again. This can be a waste of time, however, if you've got a system that is perfectly clean and works properly.
You have been warned: It is easy enough to re-format you Harddisk, but ask yourself - Do you really want to?