Wallpaper resources:

• http://wallhype.com/
  
• http://fullhdwpp.com/
  
• http://walldash.net/
  
• http://wallbase.cc/
  

Icons customization:

CandyXP is a small program I am writing that allows you to customise the icons in Windows easily.
http://www.users.on.net/~mwilson9/candyxp/about.html

Icon resources:

• http://customize.org/browse/icons
  
• http://browse.deviantart.com/customization/icons/os/?order=9

Guides:
Make Awesome Folder Icons - Tekzilla Daily Tip

Make Your Own Custom Icon & How to Use It

Theme customization:

Theme resources:

• http://customize.org/xpthemes
  
• http://browse.deviantart.com/customization/skins/

How to Patch the uxtheme.dll File (the safe way)

Rainmeter:

Rainmeter is the best known and most popular desktop customization program for Windows. Enhance your Windows computer at home or work with skins; handy, compact applets that float freely on your desktop.
http://rainmeter.net/cms/

Guides:
Let's Learn : RainMeter