Screen stalls and graphics on games suck and slow PLS HELP!

I dont know what i did but since ive reinstalled windows xp my video or
memory isnt running right. I think its the video because I will be moving my
mouse and the screen just goes blank and stalls for a few secs. When it comes
back on its slow and if I try to move a page down it does it again. When I
try and play a game the grahics are poor and slow. Please help me out.