Microsoft keeps adding new visual gadgets to the desktop to make the Windows experience more pleasant, each of them tapping the performance of the computer. Microsoft have over the years made attempts to assure that Windows doesn't enable more gadgets than the computer can handle.

Windows XP:

• Control Panel -> Display Properties -> Appearance-tab -> Effects-button
• Control Panel -> System -> Advanced-tab -> Performance Settings-button -> Visual Effects-tab.
  
  Should consider to only check the following items:
  • Show shadows under menus
  • Show shadows under mouse pointer
  • Show translucent selection rectangle
  • Show window contents while dragging
  • Smooth edges of screen fonts
  • Use drop shadows for icon labels on desktop
  • Use visual styles on windows and buttons

Windows 2000 / 9x:

• Control Panel -> Display Properties -> Effects-tab

Note it can also be a good idea to disable any background images on the desktop, as it both uses memory and takes CPU time. To change the wallpaper open Control Panel -> Background-tab.

Note if the computer is very slow, then one can consider to disable the use of True Type fonts, where each letter is rendered (Becomes worse if also using font smoothing or ClearType). Instead one can use the system fonts like "MS Sanf Serif" (Not Microsoft Sanf Serif) which has the file extension ".fon" (See Control Panel -> Fonts). To change the fonts of the current desktop theme open Control Panel -> Display Properties -> Appearence-tab.