My laptop keeps restarting itself when not booted up in safe mode?

When I turn my laptop on in normal mode it lets me log in then loads up my start bar but none of my saved icons on my desktop then goes black and restarts straight away. I can boot up my laptop in safe mode with networking and cmd fine but when ever I boot it up normally it doesn't work. I am 100% sure this is not down to my RAM or anything only thing that can be the problem is my hard drive is now faulty after 3 years or I have a virus or something? I had booted up another hard drive on this laptop and it has worked fine but the one that isn't working has a lot of things on it that I need and I was planing on selling the other hard drive :/ Can anyone think of anything I can do to fix this or find out what the problem is? I think I have given you all the details you need. The hard drive is a hitachi if that makes any difference. Thank you for your time, much appreciated.