You should always remove the old drivers completely before installing newer drivers otherwise you may get problems. I always use add/remove progs to uninstall drivers and then run a small programme called NVCleaner this shows you the residue that is left from the old drivers in the Registry and Windows/System folder before it removes them. You would be surprised how much is left from the old drivers when using NVCleaner.
It works with all versions of Windows including XP and I never have problems installing new drivers. I can't remember where I got it from but it is only a small programme so if anyone wants it, e-mail me at firstname.lastname@example.org
and I'll send it to you.