I don't know a lot about CS configs, but I'd suggest putting Source back to all the default settings and see what happens there. I'd expect that when you switch from intel to AMD some of the settings would need to change, you might want to double-check that all your instruction sets are right (MMX, SSE, 3DNOW!, etc.). I'd imagine that with a high-end video card like you've got you can probably run DirectX at 9.0 instead of 8.1.
OK, now for the joking part:
Obviously you need a new HDD. HL2 is gonna take way more than 120MB, I think it installs well over 3GB .