Well we all know that you have to back up your rank folder otherwise you'll lose everything upon a format or reinstall. However I happen to be the unlucky one to discover if you change OS you lose your stats. Even if you back them up! This is how it happened. I was running Vista 32bit and playing a lot of COD4. Then one day I decided to update to Vista 64. So I copied the rank folder over without issue and played for a few months. Ran great! I had just about everything unlocked and was a lvl 55. Then one day I ended up having to reinstall my OS (another story) and the only disk I had was the original Vista 32bit. So all I had to do is back up the rank folder and I should be good to go right? WRONG! Once I reinstalled everything and opened up COD4 I had a bad surprise. I was back at lvl 33 with over half my achievements gone! I couldn't figure it out until I looked at some old notes I had and realized this was my exact rank before the 64bit upgrade! So there you go. If you change versions of OS you are going to lose your rank. Unless someone knows better?