well....for your first problem you can just play the game in a lower difficulty.
Ex. Cod 4 or 5 for instance. If you play on regular, they are just like normal beings/enemy's
but if you play on veteran...shit....they have bloody auto lock systems, better weapons etc etc.
So just play on a different (lower) difficulty and you might find the game more...pleasurable
2. Im afraid thats how it is, but you can upgrade their AI by downloading a mod
3.cant help you with that
4.you may just decrease the volume system in the options menu
5.You need to get a better computer i,e: better video card (atleast GPU NVIDIA Ge force 8800 or better), better processor etc
Try decreasing the video quality for less problems.
6. Play it on the hardest difficulty....that will drive you mad.