For those unaware, all the hacks in Call of Duty 4: Modern Warfare have caused a lot of people to go into negative XP which also prevented them from gaining XP in public matches. This new tool that just went live now allows players to ask for a full stat reset. Note: This is for Call of Duty 4: Modern Warfare on Xbox 360 only. The new site is now live on the Activision Support site for fans to use.
This tool is for Call of Duty 4: Modern Warfare players who currently have negative stats. This indicates that your stats have been corrupted. Use this tool to request to have your stats reset so you can play online again. Requests are typically processed within 3 business days.
Please note that once you have chosen to have your stats reset, it cannot be reversed. You will start at Level 1 as though you have never played the game.
You can access the site here: https://support.activision.com/cod4tool
SOURCE: Activision Support