I had a similar problem which is well explained in two contributions to the Dell laptop forum (#19783976 & #19794919, or search on Vostro 3700 NVIDIA). It is as you think, since last October Dell has been shipping with dual video drivers and under XP they have no driver to enabling switching to the NVIDIA card. For my purposes the Intel HD Graphics driver works fine, I downloaded it from the Intel site (two drivers seemed similar, I took the most recent one).
BUT: I have everything working under XP except my firewire which crashes the computer when called from Windows (works fine under Ubuntu). I can't find any postings on this, have tried clean boot but still crashes. Is your firewire working. Do you have SP2 or SP3 (I have SP3 and am thinking of downgrading to see if that helps)? Thanks, John