Yeah, installing the codec packs does change things. If you installed the codec pack after purevideo, the codec pack might take higher priority and your videos will be played through the codec pack instead of the nvidia codec in purevideo. And vice versa, if you installed purevideo after the codec pack, the purevideo one will take priority.
To find out which codec your system is using, try GSpot.
Its all pretty complex. The forums at
videohelp are pretty good for codec info.