There is a fairly recent case law on this - Mitsubishi finance vs someone.
Basically it falls down to good title - ie who actually owns the vehicle.
In the eyes of the law, the finance company oown this vehicle until the agreement is paid in full. However, I come across so many of these where the finance company couldn't care less who has the car as the payments are still being made it's unbelievable.
On the flip side, the finance companies occasionally take some time trying to track down the car in question (to reposess it), and it can really bite you in the a55 if you;re unlucky enough to have purchased it.
Basically, don't touch it - if you (god forbid) have an accident and the car is written off, your insurers are obliged to pay the finance first (regardless of whether it's in your name), or the finance company could simply take it away from you.