So you had Vista on your notebook to start with and then upgraded to Win 7? You say you re-installed Windows 7 on 8th May. I'm not familiar with upgrading from Vista as I never used it I came straight from XP but the upgrading may be the issue. Oerhaps some other member more learned than I would come in here. If it were left to me I would download and install the very good freebie Media Player Classic. If that doesn't play your videos well then the player is not the problem.
EDIT Sorry Arc, we crossed I was slow in typing