XP Screen Locks up!

Using XP. ABIT KG7 mobo,AMD1800,Nvidia GeForce 3,wireless mose system. Something causes screen freezes when running mpeg vidieos or with sevreal web-sites open. Repalced Crucial Technologies Ram Sticks (256mb each). Reseated video card and cpu.Updated CMOS, loaded all latest WQHL drivers for mobo, VIA chipset, and video card. Still locks up. Also downloadedand instaled all XP critical updates.
Please help.