Is it only certain branches of the tree? Certain types of drives? Or folders that contain certain files (e.g. archive files, which get scanned by the tree and will also involve a virus scanner)?
Were you fully exiting Opus, and making sure dopus.exe was gone from Task Manager's process list, or just closing the window and re-opening it?
Try turning things off under Preferences / Folder Tree / Contents (expecially Zip files and other archives) to see if it's related to any particular type of tree item.