Deployment based on .NET

My question or more so a issue.

In an environment where one machine runs Windows XP and has access to the internet but is restricted due to a work environment and runs .NET 3.5

Whilst the other runs Windows 7 and runs .NET 4.5 and still restricted.

Running or adding the .NET to the setup is not available since running any updates is restricted, I found myself not being able to update the client through the internet due to restrictions on a certain port blocked.

The Issue:

When I go to deploy the application I have to create 2 builds, one for the Windows 7 and the other for XP. Is there a way for me to deploy these so I dont have to go back and forth. If Windows 7 has 4.5 surely it would just run the .NET 3.5 application, is there an easier method to this madness of mine?

I haven't researched the state of affairs with regards to different .NET versions for a while but, while .NET 3.0 and 3.5 did simply extend .NET 2.0, .NET 4.0 introduced a new implementation of the CLR, so it may be that you cannot fall back further than 4.0 in an app written for 4.5. If you build against 4.5 but only use features that are available in 3.5, what actually happens if you try to run against 3.5?