I have seen the same issue a couple of times: when docked to the left of the screen (as I have it) the taskbar seems to get a shadow that doesn’t auto-hide with the rest of the bar, covering applications on the side of the screen. This appears to happen after manually resizing the task bar (to get three rows of indicators or larger icons, for example).

On the netbook this is particularly annoying. However, it is easily fixable. Right click on a non-program part of the taskbar and untick “Lock the taskbar”. Then drag it to dock at the bottom and then back up to the left. Then tick “Lock the taskbar” and you are sorted. Thanks to My Digital Life for the suggestion (although related to a different issue).