ATI Mobility Vista Drivers and Videos

To all

I have a year and a half old computer with a 1.6 Dothlan (Pentium M rev just
before Core) and 2 gig of pc 4200 ram (the fastest it can take) with a
mobility radeon X300 with 64 mb of ram. My computer will not play standard
def videos at windowed resolutions greater than 640x480 when my computer is
running at 1920x1200. This is a problem regardless of whether my computer
is running with aero glass or basic (read XP style visuals). It would
perform perfectly when I was running XP. Note that I experience no hiccups
when I run at 1280x800 and play a full screen video. I am running ATI's 7.2
drivers (the most current version available for the mobility radeon).

Could this be the FFDShow codec? Is it most certainly the video card
drivers or does Vista have a new, more CPU intensive method to display
video?

I was wondering if there was any hope for my computer to exhibit XP style
performance in the playback of videos. I was also wondering if there was
any possibility that Dell would ever release updated drivers (7.4+) or
whether their track record is simply driver stagnation in the video realm.

Ben