Hover buttons use java (more specifically, they are java applets which use
java virtual machine from Sun MicroSystems) so if the user doesn't have
java, they won't see the buttons. Java is not included in Windows XP
(remember the law suit from Sun Systems against Microsoft, forcing MS to
dropits own JVM and not include it it Windows).
Provide an alternative (text link) navigation as well, or use javascript
rollover buttons, so even if JS is turned of, the buttons will still show
since they are displayed with the usual <img src >tag