Virus Definition Folder: 26 GB!!

Hi,

 

while trying to clean up my mother's near-full 80 GB HDD, I discovered that Norton's Virus Definition folder (Program Files\Common Files\Symantec Shared\Virusdefs) alone contains a whopping 26 GB of data.

 

Is this normal and what can I do to fix it? I don't think I should just delete the folder's content, obviously.

 

Program is Norton Internet Security Online 9.1.1.7 for T-Online (German provider) with SKU 10480989-GE.

Antivirus is 12.8.0.4 for T-Online.

 

Thank you in advance for your help

 

edit: OS is Windows XP.