If yeh already set the graphics to a minimum set maxfps 20 maxfpsbk 5, not running anything else in the background, get rid off addons, set vista up for performance.

Its due to not having enough memory. I havent check the memory usage of late but I think it averages about 600 megs times that by 5 = 3 gigs vista really needs 2 gigs to itself so everytime something else gets loaded new scenery/npcs what not something gotta get paged out to the hard drive which cause the lag.