This sounds like a driver problem. Nvidia has put out beta driver 174.74. Boot into safe mode. remove all nvidia drives with a driver sweeper program. Install the 174.74 driver (I am pretty sure it is for your card), reboot normally. I had to remove and install new nvidia and creative drivers that came out after SP1 due to incompatibility. I am running Vista 64 with a 8800gt that had same problem with SP1. My 8800GTS card I tried also lcoked up until I installed most recent driver release.