hi,
Tell that to my computer, it still needs a parity bit even if it is not used.

May I tell it to you instead? Just go to preferences and disable this feature (Device Manager ->COM port ->Settings -> Parity -> NO. BTW: it is DEFAULT settings of Billdows systems :)
I made many projects with C51 and some of them have communicated with PC. You know, none of them use parity bit. Well, there are two projects I made, which utilize this bit - not as parity one but as "extra" flag instead.

So, where does your computer still use parity bit for?

Regards,
Oleg