Hi there! Because the forum you originally posted in is for beginners trying to learn the basics of Photoshop, I moved your question to the Bridge General Discussion forum, where you'll get more specialized help.
What OS? In your Program (Library?) file do you have CS2, CS3, CS5?
CS5 is over 2 years old so when did you "upgrade"?
One other thought. If you have CS3 set to "launch on startup" in advanced preferences it will be running in the background. Since only one version can run at the same time you have to stop this process to launch CS5.