I had to strongly disagree Harm Millaard's severe warning. I've used K-lite codec packs for years and never had any troubles. Of corse commercial software producers allways have warnings against freeware, they are making their money by selling programs and codecs. Its just nonsense to talk about boot disk reformatting and reinstalling OS. K-lite codec pack is very simple to install and uninstall if needed. It has nothing to do with boot sectors and can't harm any Windows system. I think that his warning is a tendentious attack against free open software developers, who are a thread to commercial companies.
Money talks!