My media players (any) would not play avi files right. The video plays like 2 seconds faster than audio; they're not synchronized. This only happens on avi files, other format plays fine. What is wrong with it? I use K-Lite Mega Codec Pack if that matters. Thank you very much for your replies.

try to uninstall the codec pak, reboot and reinstall!!

Yes, I have tried uninstalled, rebooted and reinstalled and still did not correct the problem. Any other suggestions? Please help.

No, reinstall won't work. Either the source file wasn't encoded properly introducing a lag or due to the vid compression/player you are using.

Knowing it's an AVI doesn't give us enough info. Likely uses XVID or DIVX compression.
Using DIVx player, Zoom or MPC player (other then Windows Media Player) usually corrects the problem.
Anyway this topic about sync and lag problems has been covered many times.
Try doing a search on forum for possible solutions.