It would be easiest and betterrt to buy a copy of the FS program on Steam. The Steam version is cheap and has had a lot of minor fixes and especially graphic improvemets.
If you keep trying to install the original discs you will still have problems in getting it to authenticate.
It is well worth doing this as FSX still has a lot more than MSFS especially in the way of fairly realistic airliners.
It is not a bad idea though to have a go installing the new one too. MSFS is available on the XBOX game pass and as this costs only one dollar or pound for the first month and about $4 a month after and can be stopped at any time is worth a look and is less hardware intensive that you may think....at the moment though it is outstanding for GA aircraft but not for airliners.