According to the Valve support people if have nforce firewall it causes the problem. If you uninstall Nvidia Network Manager(or whatever the f it's called) it should fix the problem.
It worked for me, i can now play ep2 with everything maxed, including aa :)
Hope this helps
I just started having the same problem. I hadn't played ep2 in quite a while and when I loaded a save game i got really bad stuttering and the audio kept on screwing up.