Go into your Program Files, into the ET Main sub folder and find a file called ET Key. Copy it every now and again and back it up somewhere safe.
This is the file where all your game XP points etc, are stored. So the next time it goes tits up just reinstall the game and overwrite the ET Key with the one you copied.
Bobs your uncle and you will have your old rank and XP points back again.
Same thing can be done by copying the ET Main file or even the whole game if your unsure how to go about finding a specific file.
This is also a method of transferring your XP points to another PC running the game if your lucky enough to have two PC's up to the task.
Happy Gaming!