Startup Does Not Complete

I've started having a problem on my Dell desktop PC with Windows 7 Home Premium. Sometimes, when I attempt to start the PC, the process will not finish and it just hangs. It seems to happen at the same point in the process. After the welcome screen, the wallpaper and shortcut icons load. If they load all at the same time and the tray items show, the start up will be successful.However, sometimes the process stops right after the wallpaper loads. Sometimes some of the icons will appear with others as only white icons. Sometimes, none of the icons will appear. If I let it set for awhile, nothing happens - the blue circle continues to spin, but nothing else happens. When I attempt to shut down or click any of the other icons in the quick start tray, the blue circle just shows on top of them. No matter what I try at this point, nothing works. The only thing I can do is hold down the power button to shut down. Then, when I try to startup again, I get the error message that Windows was not properly shut down and I either "start windows normally" or go into "safe mode". I haven't installed any hardware since this started, and I've tried to isolate if a certain program has caused this. Sometimes I will uninstall the last program installed while I am in safe mode, but I find that this was not the problem program. When I shut down from safe mode, Windows closes properly and then I can try and startup normally again. This is sometimes successful, sometimes not.I've tried doing a full scan with Norton Internet Security and Malwarebytes while in safe mode, but they both show no issues. I also tried a startup repair in Windows, but that also showed no issues.