XP supports all versions of the .NET framework.
For Mac OS and Linux you can use
Mono[
^].
However the current Mono version does not support .NET 4.0. You will have to wait for Mono 2.12 which is
planned[
^] for the fall of 2011.
So the question is are you able to target .NET 3.5, in which case you can make use of Mono right now.