Use a text editor (such as NOTEPAD) to edit the "defuser.ini" file in the "\EA games\harry potter and the chamber of secrets\system" folder. Scroll down to the end of the file to find the following text:
[HGame.Harry]
bInvertBroomPitch=False
bAutoCenterCamera=True
bMoveWhileCasting=True
bAutoQuaff=True
fDamageMultiplier_Easy=1.2
fDamageMultiplier_Medium=2.0
fDamageMultiplier_Hard=3.0

You can change the values of "fDamageMultiplier_Easy", "fDamageMultiplier_Medium" or "fDamageMultiplier_Hard" (depending on the level you have selected) to 0.1 to take less damage.

WARNING!: Always make a backup before overwriting game files.