You are correct in stating that there is an advantage if the user doesn't have to install any other framework in order for your application to run. This speeds up deployment/installation.
The versions of the .net framework that are pre-installed on windows are covered on this page:
What version of the .NET framework is installed on Windows XP, Vista, and 7?
And also on the Wikipedia page:
.Net Framework release history
Another good reference is here:
What version of the .NET Framework is included in what version of the OS?
Note that if you plan to target users that have Windows XP then the highest version of the .net framework you can use is 4.0 as 4.5 dropped support for XP. If you don't care about XP i would personally suggest you just use the latest.
If you want the highest possible number of users to be able to use you app then the highest windows share is Windows 7 at 42% (reference: Usage Share of Operating systems) this has .Net 3.5 installed out of the box, so targetting that would be a good choice because you also don't cut off the 6.5% of Windows XP users (although they will have to install it)