Windows Movie Maker - screen goes nuts

Using Movie Maker v6.0.6000 in Vista, I was in the middle of making up a movie from around 12 short clips. While dragging the clips to the storyboard the screen display goes haywire - difficult to describe but most of it went black with blocks of barely recognisable widows and buttons dotted around. If you can imagine the Movie Maker layout, then cover it with a black sheet with a few random holes cut out then you may get the general idea! Moving the mouse around 'uncovers' other bits or covers up previously shown bits. I ended up having to Ctrl-Alt-Del to get task manager to do a shutdown - even then only parts of the task manager panel were visible. The PC is running a dual-core AMD Athlon with 2Gb of RAM so I'm assuming lack of 'oomph' from the PC is not the cause.
Sorry if my description is very vague but trying to describe how the screen went is tricky. :confused