This is caused by the menu transparency in RentalDesk NX. You can make them opaque (and therefore visible) with the following techniques:
1. Permanently make the RentalDesk NX menus opaque on each workstation. You do this by adding a command line switch to the shortcut that is used to open the programme.
i) Right click on the shortcut that you use and select Properties
ii) At the end of the line Target field that shows the programme that the shortcut will tried to run, enter the word FULLMENU, e.g.
"C:\Program Files\RentalDesk NX\RentalDesk.exe" FULLMENU
iii) Click Apply and close the dialog. Whenever you run RentalDesk NX from this shortcut henceforth, the menus will be opaque.
2. If you quite like the transparent menus in RentalDesk NX you can just set them to be opaque while the programme is running. Just use the keyboard combination, Ctrl + R