I would guess it's how handy you are writing batch files, renaming files and setting icons on the desktop?
All control settings are kept in a particular file, Standard.xml. In XP it's found here;
C:\Documents and Settings\Administrator\Application Data\Microsoft\FSX\Controls
Set things up the way you want for a particular type of flying, single, twin, B-52? Everything gets set and saved. Make a copy of the file, such as Standard.xml.single. Go back in to FSX change things how you want the next type of flight to be. Everything gets set and saved. Make a copy of the file, such as Standard.xml.twins.
Set up a batch file to rename whichever file needs to be renamed for the type of flying, then starts FSX. When FSX exits, the batch file renames the Standard.xml file back to Standard.xml.___
Set up two (or more) new icons for FSX, then edit the icon properties to call the batch file instead of FSX.exe.
Not a really difficult task, unless you have never done a batch file or edited icon properties. But it's not like things could get totally FUBAR'd.
Always remember to make a copy of the original file, just in case!