The Windows Aero Glass interface for Windows 7 or Vista requires a decent video card, you won't be able to use it on an old clunker computer. For those worried about performance, sometimes squeezing every last drop requires disabling Aero.
To disable Aero, Right click on an empty space on the Desktop and select 'Personalize'. Scroll down to the 'Basic and High Contrast Themes' section to select one of the basic themes. Simple exit the windows once you've made your selection.