Did you uninstall the card the right way ?
And installed a "Standard vga pci graphics card"
After that installed the new card , if not ?
Do it all over ........
you could have old nv.dll left on your hd
There are many ways to do this
you could install standard pci go into safe mode and delete all nv.dll´s quit and install the new one´s, i recommend www.nvidia.com detonator drivers
Disable v-sync in you open gl and d3d
In halflife use open GL and check your .cfg
is the max fps set to 74 or ? well look into it put it at 99 or something
Good Luck .