There are known problems with Icons that are 256x256 that work on Win 7, but do not work in XP. Possible issue if the icon is compressed.
The solution is to down-sample (re-create at smaller size) your Icon.
See: [
^], [
^].
For issue with icons in Vista: [
^].
You can convert Icons on-line (free) at ConvertIcon: [
^].