Oh, this is a Windows XP machine
Best answer by YegorP
FIrst of all, welcome to the Webroot Community and good question!
The reason Webroot is not backing up the .pst file is because it doesn't have that functionality at this time. However, while it doesn't sync these files directly, you can still sync them by copying them to some location outside of the Outlook folders and then changing the extension.
For more information on .pst files, I suggest you check out our "Ask Webroot" section on our support page. Just type in "pst" for the the keyword and click the "Ask Webroot" button and the article will come up! :D
Hope that helps. Let me know if you have any other questions.