Make sure to apply all the pertinent updates. Also perhaps those CS5 files were saved down from CS5.5 and contain garbage. Likewise there may be third-party plug-ins involved. Same for fonts or specific cross-platform issues. So many things that could be wrong here. Without knowing what the projects contain, impossible to diagnose. If everything fails, you can always download the CS5.5/ CS6 trial and open it there...
Duh, I just remembered I bought CS5 but had to do CS4 for After Effects because my computer is not 64bit.