I can install v550 (on Vista) which is labeled as "Setup Launcher Unicode, Text Adventures Ltd, v5.50.1.0" and all lower versions
BUT can not install v551 which is labeled as "Quest Setup, Alex Warren, v 5.5.1.0" nor any higher versions

What must I do to upgrade ?

The recent versions of quest and/or net/browsers compatibility don't work with XP anymore (I believe v520 or v530 does), so it's likely the same with Vista. Alex would be the one to help you on this stuff, if he can or is willing to try to take the time to do so...

the big message: move on, XP and Vista are ancient history (going on approx 15-20 years now!) ...
(I myself not too long ago was still using XP, lol)