codec problem

Hiya
I have a problem with watching films on my laptop (Asus L300D series). When I use Windows media player to watch movie files, the films appear far to dark (and its not possible to change brightness), whereas with my PowerDVD the same movie files appear far too bright (I have controlled with other PCs that the files are OK)! I guess this is a problem with codecs!! The problem started sometime after I had installed quicktime and DivX on my computer (now removed again, without solving the problem) probably leading to incompatibility of codecs.
Can anyone give me hint of how to solve this problem without reformatting my hard disk !