Gavin,
is it only IE8 which looks different? Does word, excel, etc look alright? If so, they may have just zoomed in/out of the browser. You can try pressing ctrl & + together or ctrl &- to zoom in/out.

Could also be what Geoff mentioned and that the display resolution has been changed. If you have a decent monitor then you normally want this setting on the highest setting.

regards Derek

+1

Same goes for if you hold CTRL and move the mouse centre wheel backwards / forwards does same thing.  Quite easy to do by mistake.