I am still seeing this issue on 4.5.2f1 using VS2012 and Windows 8 (Surface Pro 2) - Does anyone have a fix for this. Is there a timeout on how long it takes VS to open and if it fails to open in that time then opens MonoDevelop... ?? Is there a way to change that...

Tryed
renaming executables.. Failed
reinstalling and installing in different orders.. Failed
restarting... Failed
creating new project / new solutions... Failed

I have Visual Studio 2008 installed along with Unity 4.5. I had the same problem whereas in my existing Unity project, after changing my external script editor tool to 'Visual Studio 2008' (from 'MonoDevelop'), Unity will try and launch Visual Studio 2008 but then launches MonoDevelop instead. There is workaround to this. It may or may not work for you but it worked for me.

The problem:
When you use MonoDevelop for your project for the first time, MonoDevelop creates two .sln files in your project root directory and they are not compatible with Visual Studio 2008. They may be compatible with later versions of Visual Studio. If you try double clicking one of the two .sln files, Visual Studio 2008 doesn't get launched.

Solution:
1. Create a new project, change preference to 'Visual Studio 2008' before creating / editing a script.
2. For an existing project that used MonoDevelop in the past - first, manually launch Visual Studio 2008, create a new solution, and then add the existing {project name}-vs.csproj file that are located in the root directory of your Unity project. Save the solution and replace the both .sln files (you might want to rename the existing ones in case things goes wrong). I'm not sure which .sln file is associated to VS or MonoDevelop so I replaced both. You'll find that MonoDevelop will still be able to launch those Visual Studio 2008 .sln files.

Hope this helps.

Version 4.5.2: still an issue, see case 608211.

Update - apperantly, repairing VS2013 fixed the issue completely.

I've got the same problem: first I used monodevelop (worked fine) but after I installed VS2013 it switched to it automatically. After that I wasn't able to start Monodevelop at all (using preferences >> External Tools). Now it went to VS2010 (also automatically after I installed MS SQL Server 2012) and does not want to use any library references, which is the biggest problem for me.

Renaming file devenv.exe in VS2010 IDE map led to .cs files being opened in NotePad.... (Notepad!!!)
Making a copy of devenv.exe from VS2013 map and placing it to the 2010 map led to VS2013 being loaded without any files opened in it.
Navigating to another files through Browse menu or choosing other Script Editors from preferences does absolutely nothing. Its like it doesn't even work at all.

The only way for me to work now is navigating to the project folder and opening solution with VS2013 manually. Anything else simply didn't fix it (I also tried the thingy with bat file).

The problem started yesterday for me. After I did buy my pro license and installing 4.5.1f3.
I was still using 4.3.4 until yesterday. I even am considering switching back to that version, because I really can't stand monodevelop. I am sure its a good editor, but I am used to VS2010 for my job.
I hope the problem will be solved soon, so I can install the new version again.

I have VS 2012 and MonoDevelop. Monodevelop fails to load *.csproj file, so I changed it to VS 2012, but instead of it Unity launches 2010 Shell(!), which says "Invalid license" and after that Unity freezes forever.

So, in my case, the problem started after I installed an update to VS2010. Today I installed an update for VS2013 and now it Unity opens VS2013 properly again. I have no idea what the underlying issue was but it seems to be fixed for me now. I hope it stays that way.

It seems to depend on how long VS is taking to open. If VS is open already, it works properly for me, but if not, occasionally it will open MonoDevelop instead