bad performance is not lag, get your info straight
its called fps lag, get your info straight
but anyway it sounds like your pc isnt handling it properly
obviously..
set everything to minimum, lowest resolution. shadows off etc.. disable vsync and disable mouse acceleration. then test how it runs. posting your specs would also help
go to (vista) documents/my games/skyrim. open skyrimprefs and search for the line "iPresentInterval=1" change 1 to 0, this disables vsync and will improve your framerate. search for the line "bMouseAcceleration" and change this to 0
there are also mods that might increase performance