First, what media player are you using?
Even if you have K-lite, some .flv files might not play with WMP, for example. From what it sounds like you are having problems with .avi files and .flv files, so what I would do is: uninstall whatever codec pack you currently have, go to K-lite and download/install the standard 32-bit pack for Win7 - also download and install the 64 bit version of k-lite (for win7 64 bit users you need both for WMP/IE.) If that works for all files in WMP but .flv, I would download a free .flv player and use that for those files (even though win7 64 is 64 bit, many programs that it runs are still 32 bit so a free player should work.)
K-lite Standard
K-lite 64-bit
Good luck