Half Life 2

Hello,
this is going to be my first post on this awesome looking forum.
I recently installed half life 2 and lost coast(demo). the problem is...some times, all the enemies and choppers and creatures etc. get stuck...they stand still and are inactive. only i am functioning. this happens often when the game was going smoothly. there was no prob during installation either. My system is pretty old. Intel 3.0 gpu, 1GB ram, N vidia ge force 6200 256Mb graphics card. bt they are good enough requirements 4 this game i think.pls help.#some times when i repair the game, it works bt after sometime the same prob comes again. thanx ..