Terry is having problems booting up his Mac unless it's in safe mode. Leo says that it sounds like there's a corrupted kernel that is clobbering everything. He recommends backing up his data, wiping the drive and then reinstalling the OS and programs. The Mac makes it easy to do, too. If he bought his programs in the Mac App Store, he can just install all the apps he owns. Terry shouldn't install anything he doesn't need. It's better to just install as he goes. It's also an indication that his hard drive may be going bad, so it may be time to get a new drive.
Terry could also look into his boot log to see where the problem is. That could eliminate the need to reinstall everything. He can go into Verbose Startup Mode by holding down CMD+V on startup. Then, when it crashes, he'll get a detailed log based on the crash. He can then read the log and one of the last lines before the crash will give him the culprit.