Zoom going crazy! Won't stay small...

I've about scratched a hole in my head on this one, helping someone w/new (3
mo. old) Dell notebook, Vista home premium preinstalled, new Dell external
monitor. When using Dell's Fn + F8 key command to switch display to external
display, the zoom goes crazy -- desktop icons zoom to large, and if you set
them back, they automatically zoom large again (resolution is not changing,
just icons zooming large). Same is true for icons in a window, or the
contents of web page in IE -- zoom immediately goes to 1000% there. Dell had
no answers to this. Any ideas? Thanks.