Flavius

For XP MS says When you try to install an update for the .NET Framework 1.0, 1.1, 2.0, 3.0, or 3.5, you may receive Windows Update error code "0x643" or Windows Installer error code "1603"
So in Vista you have uninstall NET Framework 3.5,clear orphans by using tool Uninstall Microsoft .NET Framework with Aaron Stebner Cleanup Tool » Raymond.CC Blog

and reinstall NET Framework 3.5