Rainmeter is a great tool for customizing your desktop.
This Rainmeter theme is great for Pokémon fans, as it has all the Pokémon up to #649 (Genesect).
The skin, which can be found here, has battery, Wifi, CPU and RAM meters, and also has an email and launcher skin. You can change the Pokémon shown to any Pokémon up to Genesect, which is number 649 on the Pokédex. The Pokémon can be a shiny Pokémon or a normal Pokémon.
To change the default Pokémon shown, simply change the ‘Poke’ (located at the bottom of each section) to the Pokédex number of the Pokémon you want. If you want the Pokémon to be shiny, simply change the ‘Shiny’ (located above the ‘Poke’) to yes.
The background can be found here.