Please download the trial versions of Marine Aquarium and Goldfish Aquarium and install them. If both of them run ok, things get weird, but maybe it's like Feldon31 says and Sharks is a little too much for your built-in graphics chip. The solution is to buy a new graphics card (unless you're using a laptop). Depending on what kind of graphics port you have (PCI/AGP/PCI-Express) you have to get a matching card, but otherwise it shouldn't cost you more than ~$40 to get a card that can handle 3D alright. These guys will fill you in while I'm away.