FPS 4-8 in-game (CS 1.6)

Hi,

after I started messing with my Hz, putting it from 60 to 75, my FPS has dropped from 100 to 4-8. Even when I revert back to 60 Hz, the FPS in-game is still 4-8. I'm running CS at 800x600, OpenGL. And I doubt it's my PC, I've been running it at 100 FPS since I started playing with this PC.