The same thing happened to my notebook, which runs on Windows Vista. I believe this was due to the my deleting certain programs or files without realizing they were related to "Windows Aero". In my case my taskbar changed from some dark color (I think it was dark blue or black) to very light grey.
a) Right click on your Desktop
b) Click on Window Color and Appearance
c) Check to see if there is "Windows Aero" in the "Color scheme" menu of the Appearance Settings window. Chances are it's missing.
d) Go to http://support.microsoft.com/mats/aero_effects_not_working/en-us
and click on "Run now"
e) Once the program is downloaded and run, you would be able to change the taskbar color. In my case the program automatically changed my taskbar color back to black/dark grey.
Hope that helps