The computer can be slow because of other hardware used for the computer, not as good, or maybe it's just crammed full with things on the hard drive. Since you do tell it looks all blocky, it's most likely the graphic card itself that's old, or just doesn't have the correct drivers for it.
Try changing the rendering submode in the options menu first from OpenGL to Direct3D9 or other way around, some rendering modes work better than others depending on which graphics card.
If no luck, do as you probably already have, uninstall the game COMPLETELY(do it via the control panel), then re install it with NEW installers and not the same ones you used the first time. Then do the change of rendering modes again.
If no change, you should try installing new drivers for your graphics card. It can also help in some cases.
And if that fails, and the game is still slow and blocky, bets are that it's simply too old, or it can't handle the game. Only thing you can do then is pretty much lower the screen resolutions, no AA, no water reflections, low view distance, etc. To make the game as low demanding as possible for the computer and it's hardware.