Whats the best way to do this on windows? Do I have to use CreateProcess and then quit the game? How do i detect when the game has actually shut down so i can delete / modify the executable?

Could I just download an updater executable, do a system("updater.exe"); and then quit? How do I get the system call to not block?

Anybody done this sort of thing before?

My suggestion:

Have two exes.
front.exe - what the user loads.
game.exe - the actual game.

If you download an update, the game.exe stores the update in the current directory and then exits. My suggestion would be that the update is a zip file.

Front does something like this:

while(1)
{
    RunGame();
    if(UpdateIsWaiting())
         InstallUpdate())
    else
         break;
}

Also, you probably want shellexecute over system. System may produce a "dos" box with bad side effects.

You need to spawn the updater as an atexit, setting your program to not wait for a return...I knew how to do this back in DOS but haven't used spawn for a long time (not sure how the allegro one is set up)

I see no problem erasing the .exe while it's still running. It wont actually be erased until the program ends.

and what about using exec? That will replace the process with another so you can download updater and then use something like exec("updater.exe")

exec is POSIX.

I haven't been following the discussion, so this might have been suggested already. But what about having a small file gamename.exe which only executes (when the user starts the game) gamename<versionnr>.exe and exists immediately without waiting for a return value. Then that file could be overwritten by the updater telling it to run the new version next time. And the next time the game is executed the new version does some cleaning.