The most likely reason is that you’re missing .NET 4.5. There might be other things missing or out of date as well. Windows XP was officially retired over 2 years ago I’m afraid. Microsoft does not support Windows XP, nor do we officially.
That being said, I know that it’s possible to run AxCrypt on XP, but you may need some updates in addition to the .NET framework.
Nevertheless, I really, really, recommend you to upgrade your operating system!