Close Sonar, then go to Windows start menu:
start > run... type regedit into the box, click OK. When the registry editor opens, navigate to that key in the left pane - it's like a path in Windows explorer. Click the "SnapDlg" key (folder), and you'll see the values for LastX and LastY in the right-hand pane. These are the screen coordinates for the dialogue box in Sonar. Double-click each of them and type in some low value, so the snap dialogue will be set near the upper left corner. Close the editor and start Sonar.
IMPORTANT: Make sure you're editing the right key. Better make a back-up of the registry, just in case... (File > export...)