I'm posting this for a friend, so I don't know all the details, but if there are any questions I can find out.
He has a HP Pavillion a6750f. He just reinstalled Windows 8.1 (possibly an upgrade from 8.0 - his brother did it and he's not sure) and suddenly has low (like 8-9) FPS. I thought it might be a driver issue, but unfortunately HP doesn't support Windows 8 on that PC. Still, he has some installed (most likely automatically) and I'm thinking that's not the issue.
He also tried reinstalling ET (with a different installer), doing a cvar_restart, and r_primitives 2, as well as some other things I'm sure. It's only taking 20% of his CPU according to task manager, and it's certainly not a memory issue. The sound is also laggy, as is expected with such low FPS. I can't think of anything else to try, so I bring it to you. Thanks for any help!