I'm sorry, I didn't see your post before, so this answer is a bit late. But it might clarify some things regarding Visual Studio 2012 / 2010.
- Visual Studios do not break each other. I successfully worked with parallel installations of VS2003/2005/2008, VS2008/2010 and currently VS2010/2012. There is no problem whatsoever doing that, and never has been.
- There is absolutely no issues at all with .NET 4.0 apps if you install VS2012 alongside VS2010. You can do it if you like and use both Visual Studios. Also, you can tell VS2012 to create apps for .NET 4.0, no problems there either.
So, don't be afraid, everything will work fine ;-).