Normally, each time you right-click a file, select Open With... and choose a new program from the programs list, it will append itself into the pop-out menu for future use. I don't quite remember when this stopped happening, but it now forces me to browse through my Open With... list each time I want to open a certain file with a program other than its default association.
For example, I have an AVS script that opens by default in Windows Media Player, often I would like to edit this in Wordpad, so I right-click it and select Open With... and choose Wordpad. In normal circumstances, after I've done this the first time, Wordpad should be now on the Open With... drop down menu. Instead, I am forced to manually select it each time. Keep in mind that what the applications added prior to this error occurring are still there, i.e. Windows Media Player will be on the list, but Wordpad will not appear.