Windows XP reached end of life in April 2014 and is neither supported by Dell or Microsoft.
If you need to run an app that requires the .NET Framework 4.5 or later, you will to upgrade to a later version of Windows or find another machine that has a later version of Windows. You can also contact the publisher of the app, to see if they support Windows XP with another version of their app.
The .NET Framework 4 is supported on Windows XP, however, it will not run apps that require .NET Framework 4.5 or later versions.
No. Beyond that, Windows XP has seen no security updates in almost four years - it's a sitting duck online. If you must for some reason run XP, keep it disconnected from the Internet - for both your own safety and security and those of others.