The only way to reinstall the application is through the download package available on the official website. Firstly, go to Start > All Programs > Internet Download manager > Uninstall IDM. Secondly, choose FULL and let it delete everything.
Note: Make sure you have all the browsers closed all the time.
Once the software is deleted from your computer, restart it and then download the package from the official website.
Download IDM: https://www.internetdownloadmanager.com/download.html
Install the package again and then register it with you details.