Query Folder of Xml files

Question

I am using .xml files to store message threads(as in forum thread) for a network based messaging program. One folder holds all of the files, each user queries all of the files in that folder to check for updates. This takes forever every time a user updates a file or checks for new messages. Is there a better way to do what I'm doing in C# using XML or should I just learn SQL? Thanks for your input!