I'm not expecting much here, but try display properties, appearance tab, press advanced. Then the two items 'Active Title Bar' (should be size 20 font Trebuchet MS) and 'Active Window Border' (should be 1). Then on the Settings tab, press advanced and check the General tab there, and DPI sttting should be 'normal size 96dpi'
I have never come across anything directly controlling the size of icons in the task bar, so you should really suspect some rather silly virus has been caught