We all know that Windows 8 is under development and many interesting features have been revealed in Windows 8 like new Start Screen, Aero Lite theme, User Picture Tile in System Tray, Aero Auto-Colorization feature, etc. You can change your windows xp theme to windows 8 theme. it includes Color appearance, Sounds, Icons, Fonts, Login screen and aero features.
I've posted few screen shots of windows 8 theme below. you can download theme pack for windows 7 and vista too. check those themes and download from this page
Download windows xp to windows 8 converter pack from here
Visit this page to read more about windows themes installation tips and guides.
Download Windows 7 to Windows 8 transformation pack.
NOTE: Remember to create a System Restore point first before installing the skin pack so that you can restore default files and settings in case you face any problem or you don't like the new look.
Like more customization? you can add any theme as you like to your firefox browser. you can make your own themes and use them too. it's free and no softwares needed. read how to customize your firefox browser with custom browser themes.