Icons get corrupted on Desktop and Task bar

At certain point in time especially when you are running a lot of Macros with Excel or MS Access, sometimes the registry get corrupted for no reason and this corruption affects the icons on your desktop or taks bar getting ugly (such as the 16 color mode). This is a known issue, caused by an incorrect data assigned to Shell Icon BPP registry value. Re-setting it to 16 resolves the problem. Use the following steps: Open Registry Editor and navigate to the following key: [HKEY_CURRENT_USER\Control Panel\Desktop\WindowMetrics] In the right-pane, double-click Shell Icon BPP and set it's value to 16 Close Registry Editor.

You will need to restart Windows for the changes to take effect. If that does not help, or to apply the change instantly, try this:

• Right-click Desktop, choose Properties, Appearance Tab.

• Click Advanced button, choose Icon from the drop-down list

• Set it's size to 31 and press OK, OK

• Repeat the steps (1-3) and set the Icon size to 32 (defaults)
  
  More Reference: http://windowsxp.mvps.org/repairicons.htm