MW2 isn't like CoD4, your profile is't stored on your computer, that is only for single player, spec ops and for tracking who to log in as as default.
MW2 is like Waw, your profile is stored on the IW servers and whenever you rewinstall you have a 50/50 chance of keeping your profile, how it works with you keeping it, idk, but sometimes it recognises who you are sometimes it doesn't, it is how it was in WaW, it is how it is in MW2