(FIXED)How can I get Unity 4.5 back?

I am a Unity user but it won’t last until someone helps me. I was downloading 4.6 and it messed up Unity forever… Do I have to find a way to get it to install again, or, do I need to get a new computer (That might not ever happen). Otherwise, I will have to quit Unity forever. I have tried everything but it won’t work. I am sorry but I may not be able to use Unity forever. the problem I think is that I had Unity 4.5.5, but I’m not sure yet. I need help AS SOON AS POSIBLE!!!

http://unity3d.com/unity/download/archive

I got it fixed!!! I had a simple error saying I had to uninstall it right.

Well if you ever wanted to install more than one version, you can just changed the install path, then you can experiment with multiple versions.