fdisk will not format the drive, it allows you to partition the drive. Changing the partitions will mean you lose all the data on the drive. format will format the drive, and will also mean you lose all the data on any partition you format. You should not use either of these dos utilities unless you have backed up any important files you need to save. A fresh install is usually better than an upgrade or reinstalling over the top of the current installation, unless you want to keep a lot of your current files and settings.