I've put together a short article with screen shots on resolving right-alignment issues with labels.  The solutions are simple and obvious, once you know to look for them:

1. Make sure the html element used for labels (span in this case) is large enought to handle different font settings.
2. Be aware that the font you are using in the Visual Studio 2008 designer is probably not the same for most of your users.
3. If layout is critical, you should probably specify font-family and font-size in the element's style. (However, be aware the user can override your settings.)
4. Always test your web page with an accessible theme in IE or FireFox.