Since I formated my PC I can't play Avi (I just tryed Xvid,but mpeg works fine) files properly and I know these files are good.And I never had this kind of problem in my PC (it's not a slow pc) and I don't know what can I do now.I've already tryed a lot of codec packs and players but the problem is always there

Please help me.

Exactly what hardware specs (CPU, VGA card, RAM)?
Did you install (after Operating system) latest graphics card driver, DirectX9c?
And well you shouldn't have installed a lot of codec packs. It can be a reason as well. Just the codecs you really need you know...