extremely bad game performance

I've noticed that a lot of games lately just freeze up on the opening
screen. The video is choppy and doesn't seem like it's going to work, then
it eventually "catches up" and starts working normally. This happens every
time a new screen loads, though, so the games are unplayable.

I thought maybe it was just newer games that my computer may not be strong
enough for, but I reinstalled Return to Castle Wolfenstein and it does the
same thing. That game had run perfectly a few months ago.

My graphics card is an NVIDIA RIVA TNT2 Model 64/Model 64 Pro, and I just
updated the drivers to the latest version.

Any other ideas what might be wrong?