That is part of Reflections that have been put in the Windows I have posted a screenshot which also shows them around the border of Windows Explorer and yes it does show on the taskbar to.
If you want to get rid of them I can make a custom theme for you that will remove them.
If you are using the default aero theme I can change that for you, using Windows Style Builder and post it here in a zip folder.
If you are using a custom theme you will need to ask the theme maker. It's not cool to change someone else work.
This will be a custom theme and you will need to do 1 of these 2 ways of Enabling custom themes for it to work. Enabling Custom Themes - Windows 7
Or Theme - How to Apply Custom Themes
All I need from you is a name to call it.
Let me know.