Pop it up in 'windowed' mode first to see if that fixes your problem, then ingame :
hit "U" to open the control panel, go to "mouse" (i think.. or input devices -> mouse) and switch to "Hardware Mouse".
or something similiar.. let us know what happens.
EDIT :: ooh also, make sure you have the background downloader deactivated
(Go to your ryzom folder, open "client.cfg" in notepad, and either find the BackgroundDownloader = 1; and change the 1 to a 0, or if it's not there (i dont remember if it comes default..) add BackgroundDownloader = 0; to the end of that list)
the bkgnd downloader takes attention away from ryzom for a minute, then back to it - this causes insane amounts of problems and is totally a waste of time. *coughwasn'tagoodideatoimplementcough*
Bkgnd downloader + Vista = pain.
Bkgnd downloader = pain anyway