Visio is always a paid-for extra, it's never included in any of the Office packages.
The Visio 2000 format (.vsd) was updated in Visio 2003 and a new XML format (.vdx) was added and later versions of Visio have deprecated the XML version and the original V2000 format (but not V2003), so you may have issues if you wish to update to a later version of Visio, it will be a one-way journey, no going back to Visio 2000.