I've had the exact same issue. Was happily running GTA IV on Steam through Boot Camp and after installing a couple of Windows Security Updates the frame rates fell through the floor. Even dropping settings to minium still gave a shocking performance. At first I thought it was a GTA issue, but other games had the same frame rate drop. I started to dig around for gfx driver updates but didn't want to poke around too much until I learnt more.
Now I've discovered the same issue is occuring on the Mountain Lion partition.
Running Trine 2 at full res, which was once a glorious site is also now cripplingly slow.
I've installed GFX card status to see if the Nvidia is firing up and it says that it is... but I've no way of knowing for sure.
Could the Nvidia card itself be faulty?
Odd that his happens to both of us on the same day...
OK: This worked for me:
You need reset the SMC.
You have to shutdown the computer,
make sure it's connected to power and press:
Left shift + CTRL + ALT (Option key) + PowerON Button
(It won't turn on, the power light on your cable should flash green).
Now restart it.
Trine 2 now glistens perfectly. I'll now have to see what happens on the other side...
This has happened several times now. At first only after unplugging the MBP to a TV via HDMI.
Though recently it occured spontaneously. It's annoying because you only become aware of it when firing up a particularly graphically intense program/game.
Acutally - having to reset the SMC every few days is annoying full stop.
I wonder if there is another way to see whether the Nvidia card is activated or not? (If that is the issue?)