I wanted to make a post about this I can link since people ask in Discord all the time.
Step 1: Open Windows Security and click the text in the left navigation pane that says "Virus & threat protection".
Step 2: Look towards the middle-to-bottom, and click the blue text that reads "Manage Settings" underneath the heading that reads "Virus & threat protection settings".
Step 3: Scroll to the very bottom of this page.
Step 4: Click on the blue text that reads "Add or remove exclusions" underneath the heading "Exclusions"
Step 5: Click the big gray button that says "+ Add an exclusion". Navigate to the folder that all of your PSO things are inside of. It's probably something like C:\Programs\Ultima-PSOBB or something, idk I don't use my C:\ drive.
Step 6: The folder you clicked on should appear in the list underneath that gray button now.