Windows sizes

I have a PC running Win 98 and I cannot seem to resize the windows in programs like Word or Internet Explorer to full size (resolution is 800×600 or 1024×768) When you maximise or try and drag a window from the corner it simply stops about an inch from the task bar at the bottom so you can see a strip of the desktop. In in 640×480 it will resize correctly. I have tried three different graphic cards and they all behave the same. I suspect it may be a registry error can anybody offer suggestions.It is a Celeron 1.7 256MB ram with a Sis 315 Graphics card running Win 98