Are you confusing a format with a restore (back to factory settings) - I ask the question as you say it came with Vista pre-installed which suggests to me that there is a "hidden partition" which puts everything back to how it was when it left the factory - ie: how it was when you bought it.
There are instructions here for doing that click here - it relates to a different model but the procedure should be the same.
First you need to back up all of your data - ie.documents,pictures, etc. - After a restore you will need to reinstall all of your software and windows updates plus any up-to-date drivers that you may need.
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