Computer screen choppy videos

Hello,
I just bought a new HP Compaq dc5850 Microtower for my small business. I can play videos and music, edit videos, and run photoshop and multiple applications flawlessly! BUT, when ever i try to watch a video fullscreen, it becomes choppy and splits down the middle! I want to watch youtube and netflix without the chop in full screen. It does this in my video editing software as well. i can play on the small screen but when i fullscreen it it becomes choppy. the sound works fine. My monitor is set to 1280x1024. it's a Dell flat screen but i'm not sure if its LCD or not...

I'm not a tech guy, but I wanna be! I have no idea what I'm doing here, but I purchased the HP Compaq dc5850 Microtower because I wanted to be able to upgrade its components to meet my needs!

ANY help will be greatly appreciated. Please speak in NOOB terms!

thanks