FX5700LE - extremely poor full screen video performance after MB + Processor upgrade

I just replaced my MB & Processor Abit AV8 (VIA chipset) + AMD 4200 x2,
replacing and ASUS A7N8X (Nvidia2) + AMD 3200+. Removed all the old Nvidia
drivers (except video) & installed the VIA / processor drivers. All seemed
ok except the display wasn't acting right so I uninstall video drivers,
booted with Std. VGA & then reinstalled them (tried two versions 66.x &
78.01). Multiple desktops enabled ( 2 monitors).

Everything works ok except: if I enable full screen video & play any video
(mpg, avi, dvd) (any player) the system slows to a crawl (mouse barely even
will move). With it turned off, full screen within a player is ok.

OS - XPsp2, DirectX9c.

Any ideas what might be the problem?

Thanks,
Dale