Finally tracked down the data file store for CoD4 under Vista.
C:\Users\David\AppData\Local\VirtualStore\Program Files\Activision\Call of Duty 4 - Modern Warfare\players\profiles\
Dai
So on yours the username will be different (the David bit). If you look for the USERS folder and drill down through (account name)\AppData\Local etc etc eventually you should find a profiles folder. Drop your
LittleDragon folder off the old PC into that and you should get your ranking back.