I love download managers but yeah, they don't always work well with some sites (Nvidia is one of them), what you want to do is go to Flashget options and delete all the "File Types" from the option screen, this way, flashget will not start inmediately.
I use IDM and it has a File Types Tab, under it, it says... "Automatically start Downloading the following file types:". Flashget should have something similar and you will find a bunch of different file types (ie: .exe, .zip, .rar, .doc, .pps, etc, etc, etc.) just leave it blank and save or hit ok, next time around, it will prompt you or you will have to right click on the file you want to download and choose to download it with Flashget.
Hope I haven't confused you more.