you are definitely pushing that system too hard with your graphics settings...the fact that you used the words "default graphics driver" leaves the assumption that you're using the graphics chip built onto the motherboard, and those chips cannot handle hi-res settings. you should probably be running 256 textures, no shadows, bump map and detailed terrain, disable other player skins, disable weapon effects, disable vehicle effects to stabilize your frame rates a bit.
didn't you say somewhere that your system is running vista? if so, there is a whole list of stuff you should disable including windows automatic updates as well as the update options for anything like itunes, acrobat reader, flash player, java, etc...and get yourself an anti-virus you can shut down before you start ah...all of those things run in the background and can cause problems.
