Clean Install Vs. Reformatting

[Windex1]

What's the difference between a clean install and a reformat?

I'm learning about upgrading computers with windows operating systems and i don't quite grasp what the clean install idea is about.

Does a person delete everything on their hard drive only keeping the operating system information, which then lets them upgrade to a new OS?

How does a person delete everything on their harddrive except the operating system information so they can upgrade a windows operating system?

 

[setishock]

Ok..

A reformat leads to a clean install. If you format the drive, it's all gone. I recomend the long format by the way.
Upgrades involve already having a OS on the drive. The upgrade package adds the files to the existing OS to make it operate as the new OS. Like upgrading 98se to XP home for exsample. I don't recomend doing upgrades as the main reason is when folks have reached the limits of the current OS or are having problems with the current OS, They just drag those problems to the resulting new OS. A clean install is the recomended method...

 

[Windex1]

How does one exactly do a clean install? I'm talking about upgrading to a 9x system, not XP.