Maybe you have a registr y problem. Hmm... That would make me think that something needs to be reset. Try right-clicking on your taskbar. A menu should appear, select properties. Then change the option "taskbar buttons" to "never combine". Press the apply button and see if that fixes the problem.
If that doesn't work I would suggest using system restore. You can find it in the start menu under accessories -> system tools. If you can't find it, type "system restore" in the search box at the bottom of the start menu. When you open the program, click on the "show more restore points" check box. It should expand the list of restore points. Choose a date from the list when the problem hadn't occurred. I would caution you that system restore will revert your computer back to the way it was exactly on the date you selected. That means that programs, apps, or changes you've made to your computer since then will be gone. Don't worry to much, you can always undo a system restore. Once you have selected a date, click next. Finally, click finish. Your computer should then log you off and start the restoration process. It may take several minutes to complete.
Good luck!
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