The “Open with” dialog box recommends programs to open a given file. The contents of the Recommended Programs list are determined in part by the registry key HKCU\Software\Microsoft\Windows\CurrentVersion\Explorer\FileExts\filetype\OpenWithList
(where filetype is the extension of the file type in question). So, for example, the …\.bmp\OpenWithList key may include several values, such as Notepad.exe. Deleting the unwanted item in the OpenWithList key will remove it from the Recommended Programs list.
Some items in the Recommended Programs and Open With lists appear as the result of values within the HCR\filetype key and its subkeys. However, accidental additions to the lists are always made in the HKCU hive.