Could you tell me what .NET framework installed by default on Windows XP system?
Because I wanted to write a C# application that does not require anything else to be installed.
If none is there any way not to force users to download .net framework (for example 3.5) but install it?
Windows XP did not come with any version of the .NET Framework.
You can distribute the .NET Framework 3.5 redistributable with your app. Get it here. Depending on the setup mechanism of your app, you will be able to set it as a dependency of your app's executables so that it's installed automatically for your users.
No .NET framework version was shipped with the original version of Windows XP. This may have changed with different service packs. Starting with Windows Server 2003, .NET Framework 1.1 was preinstalled.
来源：https://stackoverflow.com/questions/1855980/what-net-framework-shipped-with-windows-xp