at first glance...i would say that either the video card driver is corrupted or someone changed his resolution....
right click the desktop and choose properties then go to the settngs tab and monkey with the resolutioin....see if that makes things smaller...if not (or if you can't change the resolution) download a new graphix card driver from the manufacturer's website
the taskbar taking up the whole screen sounds like someone expanded it...if you hover the mouse on the top edge of that bar the pointer should change to an up and down double sided arrow...click and drag to make it smaller (if you don't get that...right click on the taskbar and untick lock the taskbar and try again)