all that you really need in an xp icon is 32bit versions of the 16x16, 24x24, 32x32 and 48x48 if you're using large icons mode

the only time you need to go into lower bit versions are when your graphics card cannot support them.

Truth is you have to learn where each icon shows up if you want to make a perfect pack. For example:

Alerts: 24/48px not needed.

Shell icons other than startmenu's: 24px not needed.

Dialogs: only 32px needed.