Shift + Tab Button Doesn't Decrease Indent In Word 2007

I recently bought a new laptop and I installed Office 2007 on it (it had already been installed on other computers before). However, pressing Shift + Tab in Word doesn't decrease indent (It does so on PowerPoint). This does so on my other computers, and I assumed it was a default function in Word 2007. I tried to customize the keyboard shortcut for it, but Word wouldn't accept Tab as a suitable shortcut key (instead of registering Tab as part of the shortcut key combination, it performed the normal Tab function and went to the next gui object). Is there any way to fix this? I can live with it, since all I have to do is make a shortcut without Tab, or just simply click the respective buttons, but the Shift + Tab shortcut is ingrained in my memory and will be hard to unlearn, especially since any other computer I use with Word will use the default shortcut.