In order to solve the issue, I suggest you to uninstall the application using Add or Remove Programs feature:
If you use Windows XP or bellow you can go to Start > Control Panel > Add or Remove Programs. In the Currently installed programs box, click on the QuickInvoice program and then click Remove.
If you have Windows Vista or above you need to go to Start > Control Panel > Programs > Programs and Features. Select the QuickInvoice application and click on Uninstall.
After that, you can visit the review page of the product in order to obtain a link to the official download. Press the Download button, save the file to your computer and reinstall it following the provided instructions. Be sure to check the Make shortcuts available to all users box instead of Install shortcuts for current user only.