It should work on both vista and xp.
(If not, make sure you have Windows XP Service Pack 3 installed through windows update as well as .NET Framework 3.5)

A few of you have been asking about the locked options, these are because of the separate versions.

Advanced
-Allows all options to be used

Premium
-Damage is restricted
-Add Scope is restricted (due to developement)

Basic
-Scope Vision is restricted
-Damage is restricted
-Range is restricted
-Add Scope is restricted (due to developement)

Note: "Add Scope" as well as certain scope mods will be added in an update so check back regularly

You can check for updates by clicking the "Help" or "Update And Support" button in-program.