just try reinstalling hl or esf (prefably hl , more likley to be the problem with the game engine) reinstall and see what happenes (both hl and esf)
Newer drivers don't always help , if you put todays latest nvidia drivers on a genric fx5200 , you will se the perfomance drop (just look at a 3dmark score , thou this wouldn't matter on the asus gaming edition fx5200 due to a reconstucted gfx board and faster memory) it could also be your xp setup , as the hl readme will tell you that virus,worms,tojans etc. can screw with your hl exe