Similar to my ET Tip, this could save you a lot of heartache ladies.
If for some reason your PC needs to be reformatted your CoD4 online account will be gone! The reason for this is because your account is stored on your PC, not online.
The best way to counter this is to periodically backup the "mpdata" file (which is only about 9 kb).
This can be found in C:\Program Files\Activision\Call of Duty 4 - Modern Warfare\players\profiles\*YourCoD4Name*
Then after you back it up, zip it and try to either put it on a disk by burning it onto a CD, or save it somewhere online. Maybe send it to your email account (which is what I do), or upload it somewhere safe. This way if you ever have to reformat your PC, you won't lose your CoD4 account containing your rank and weapon unlocks.