Simply do the following:
Control Panel > System > Advanced System Settings
Then click Settings in the Performance section.
That'll bring up the Visual Effects tab.
From the four radio buttons, choose Adjust for best performance. If you change your mind you can always go back and choose one of the other options, including the default option.
You would think that a computer with 2GB of RAM should be able to run everything smoothly, but Vista uses a huge amount of system memory, so from time to time you may want to turn off the visual effects features to speed things up.