minimum requirements for BHD is a 1.2Ghz cpu (1200Mhz), when I first started playing BHD I had the same problem with the choppy sound, which was fixed when I upgraded my cpu.
@ chief he told us most of that
Windows XP pro
384 ram
1000Mhz CPU
Video card is AGP, though video card size is unknown - all of that was in his first post.