I know this is an old thread, but it is the first result when searching "deskjet 5850 vista," so I figured this would be a good place to post my findings.
According to an
HP support article, the correct substitute is the Deskjet 5600 driver. This driver should be included with Vista (it was with my copy of Vista Business Retail), but I won't be able to try it until tonight at the earliest.
Like I said before, sorry about the bump, but I think this is pretty useful information for HP printer owners.