Yep it was something that started on XP and transitioned to 7.
Nope, we cannot make it to .NET 4.5+ due to some third party libraries, which is a bummer, because we are stuck with BackgroundWorker as TPL had some bugs with Forms on 4.0, which were only fixed on 4.5.
It is only one project thought, the other issue are the IT upgrade policies regarding developer images.
You can get async/await on .NET 4.0 using Microsoft.Bcl.Async
...But reading between the lines, you probably still have clients on XP (.NET 4.0). That's gotta sting. We made a fuss about TLS1.2 support being required and forced them to upgrade to Win7/.NET 4.5.2 at least.
I have gotten gotten a smallish winforms app to work with the new .csproj format. But it's painful. WPF is probably a nightmare.
As an aside if you make it to .NET 4.5+ and get async/await. It works great with winforms and presumably WPF!