Worth mentioning first and foremost, this is the main problem issue for newcomers. Several game updates or ‘point releases’ were issued over the life of Quake III, each one with a new network protocol. A protocol is the "language" in which the server communicates with the client. If the server uses a different protocol than your client computer, you cannot to connect to that server since the server and client don't understand each other. Mismatched clients are dumped back to the menu with the message "server uses protocol xx" where xx is the protocol number.
Quake III uses different network protocols depending on the point release used.
1.16 -> 43
1.17 -> 45
1.2x -> 48
1.30 -> 66
1.31 -> 67
1.32 -> 68 (the latest point release upon which all info in this guide supports)
See the Quake III Mods, Point Releases, Custom Maps & Utils thread for links to the latest point release. Only the latest point release is necessary and already contains updates from all previous releases.
"User Interface is version 4, expected 3"
This means your quake3.exe isn't compatible with the currently installed point release. Reinstall the 1.32 PR; if that does not work the only solution is to remove all pak*.pk3 files located in baseq3 except for pak0.pk3 and then reinstall the latest point release.