First, make sure you are downloading the latest
non-beta drivers from nvidia. (The ones I linked to are 32-bit Vista)
Next, you may want to *completely* remove your nvidia drivers by uninstalling them via Control Panel > Add Remove Programs.
Then use a driver cleaner like
Driver Sweeper to clean up what's left behind.
- Reboot your PC in Safe Mode.
- Run Driver Sweeper and select what to clean.
- Analyse lists all the entries possible to remove, cleaning removes the entries selected.
Reboot your computer again and start it in normal mode. Install the drivers you downloaded, restart again and you should be golden.
If you still get BSODs, then you should try using a different version of the driver. Perhaps even a beta video driver. There may be a better place to go, but Laptopvideo2go has plenty of beta drivers for laptops
and desktops.
If the problem *only* happens in L4D, then you may want to look into problems with L4D itself. A reinstall or update may fix your issues.