Question: I have read here <http://www.microsoft.com/downloads/en/details.aspx?FamilyID=5765d7a8-7722-4888-a970-ac39b33fd8ab> that to instal .NET 4.0 CP (client profile) you must have WinXP SP3 or above. This is somewhat worrying to me as there are many people on SP2 or below.

Is there any way I can run a .NET 4.0 app on any version of Windows xp and above? (without worrying about which service packs they have)?

If not, what about .NET 3.5, 3.0 or 2.0?

My questions are the following:

1. How low do I need to go in my .NET Framework version choice to ensure compatibility for WinXP SP2 or above?
2. How low do I need to go in my .NET Framework version choice to ensure compatibility for WinXP (no service packs) or above?

Answer: 
* .NET 4.0 requires XP SP3, Win2k3 SP2, Vista, 7, or 2008(R2)
* .NET 3.5 requires XP SP2 or newer.
* .NET 2.0 requires Win2K SP(3?) or newer.

Incidentally, XP SP2 is no longer supported. (also; all versions of Win2K, and Vista XP1)
As far as I know, 3.5 was the last version to support WinXP SP2.