There are XP based skins for window blinds here at Win Customize, but you can not use a 7 theme for XP, nor can you use the WBA file anywhere but through windowblinds.
If you are trying to open a skin to edit for xp, you need:
Skin Studio!{found at Stardock store!}
This will allow you to make a skin of your liking, for your operating system!
Hope that helps.
