Had to reinstall FS2004 becuse I was having .dll problems causing it to crash. Used the uninstaller and then cleared the FS9 directory completly. After doing that I used a register cleaner to get rid of any entries. When running the sim after reinstalling, flight sim said there was an aircraft not installed (a third party A320 I was using previously and which was the aircraft I had used in my saved flight). How did it know this as it was a complete reinstall. All my saved flights were there too. Where does it keep this information? Is it not possible to do a completely clean reinstall?

Somewhere under a "Documents and Settings" folder or sub-folder, you will find the files which retain things like saved flights, weather, etc. Obviously, since these are created _after_ the sim has been installed and used, they can not be removed by uninstalling the sim because the FS uninstall.exe just simply doesn't know they're there! This is the same reason add-on folders aren't removed during an uninstall.Your previous "saved flight" is still active and since the aircraft you had used previously in that "saved flight" is still being referred to, and is not present, you will get an error/dialog box.Saving flights with default aircraft and/or panels has always been a safer method. You can always change back to a plane of your choice _after_ accessing the sim.