When you say "startbutton is not shown on all visual styles" are you talking about the Windows Visual Styles ie: Blue/ Silver/ Green - or have you intalled a visual styles program? If you have, they try running the windows default styles and see.
have you tried other themes and tried them?
I can only guess that it might be the theme that is a problem, there are some themes that come with StyleXP - do they work?
Check for an up-date of StyleXP and perhaps let them know of the problem.