Are you trying to format your main drive within windows? Or course that cant be done.
When a format cant finish, it's usually because of a bad sector it couldnt recover or something of the like. Try to format it again. Try to format it from the command prompt, using 'Format X:/u" (X being the drive in question)
If that dosent work, try this, but only if you are comfortable with the innerds of your computer.
Take out the drive and place it in the freezer for a few hours. But not if your freezer is old and Frosts. Take it out, and make sure there is not and moisture on it. When it is still cold, plug it in and try it. You will be suprised how often that gets a drive working again.