This is caused by a Windows registry corruption and can be fixed thus:
1. Download THIS and extract the Nav_Regedit.exe in it to your RentalDesk NX folder (c:\program file\rentaldesk nx)
2. Run the programme Nav_Regedit.exe. Expand the SC UNTHORPE folder on the left, then FORMS, then select MainForm. Right click on MainForm and select Delete from the popup menu
3. Close Nav_Regedit.exe and restart RentalDesk.