Which .NET Framework files necessary?

I have the following on my computer (Windows XP), taking up much space:

Microsoft .NET Framework 1.1
Microsoft .NET Framework 2.0 Service Pack 2
Microsoft .NET Framework 3.0 Service Pack 2
Microsoft .NET Framework 3.5 SPI

Which of these — if any — do I really need? What happens if I uninstall any or all of them? I do very little development work now, and none (as far as I know) using .NET Framework in any way. Could these be used as run-time resources for other applications?

— Gary T