Go into C:\Windows\Resources\Themes and double-click the Luna theme file. (Luna is the default XP theme.) A new Theme Settings window should pop up, which you can click Apply on to load it over Windows Classic. If the Luna theme's not there or if it doesn't work let us know.
Yesterday, I unfortunately discovered that there are different downloadable themes. So after some searching, I installed one, didn't like it, changed again. I went through a few, before I decided to go back to my original XP start menu and toolbar etc. (Under Display Properties, Appearance, Windows and Buttons). But all I have now (after uninstalling all the other themes) is "Windows Classic Style". Could someone please help me get back my other Windows and Buttons. I have already system restored to before I did anything, but still only have the one option.