I recently change my desktop background and i noticed that my task bar wasn't transparent anymore. I googled the problem and found this...
When i opened the "Window Color" thing there was no check box, or "enable transparency" 
- 1 Right-click on the desktop.
- 2 Select "Personalize" from the right-click menu.
- 3 Click on the "Window Color" icon.
- 4 Click in the box next to "Enable Transparency."
- 5 Click the "Save Settings" button. The changes will take effect immediately.

My Computer
- OS
- Windows 7 Home premium 64bit
- CPU
- Intel Core i7 950
- Motherboard
- MSI X58M
- Memory
- 12 GB DDR3
- Graphics Card(s)
- Evga GTX 570