Hi,
check your nvidia drivers for something called this:
NVIDIA FXAA Technology – shader-based anti-aliasing technology available from the NVIDIA Control Panel that enables ultra-fast anti-aliasing in hundreds of PC games. FXAA delivers similar quality to 4x multi-sample antialiasing (MSAA) but is up to 60% faster, enabling substantially higher performance in games. FXAA is supported on all GeForce 8-series and later GPUs
Having the above enabled could give the issue your seeing. Try disabling it.