Is it laggy on all maps? The latest maps tend to be more demanding of the GPU. Is the performance on all maps significantly worse than it was on previous versions (like 0.5, 0.552)?
What has changed in your PC before the lagging appeared? The thread title says "Windows 8 Lags" - is that what has changed - you only installed Windows 8 and it was working fine before that?
You didn't write correctly the graphics card's name. Is it GeForce GTX 460 (192-bit)?
I have some ideas. It's possible that you get low performance due to some graphics settings. We could investigate those. Go to the directory where the game is installed, open these two files with Notepad and post the contents here:
\Battlefield 1942\Mods\bf1942\Settings\Profiles\Custom\Video.con (or whichever profile name you use instead of "Custom")
You may also get low framerates if your graphics card is not in your main PCI-e x16 slot or not working in x16 mode (it could be the case if you have more than one graphics card installed).
Are you sure your monitors are connected to this graphics card and not to the motherboard (where you may be able to use the Intel GPU integrated into the CPU)?