aalleexxx5 wrote:it says that it dosnt exist and it is too short

So it seems to be the wrong serial. The "real serial" is on the back of your CD sleeve. If you have lost this CD sleeve then your serial is lost too :-( Nothing that you can do unfortunately. I am not sure if it is possible to get the serial from your already existing installation but I guess you can not recreate it from there.
Greetings,
Steve