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 3.5 will give you compatibility with XP SP2. If you want compatibility with any level of XP, you need to go all the way back to .NET 2.0.

.NET 3.5 requirements can be found here:
<http://msdn.microsoft.com/en-us/library/cc160717(VS.90).aspx>

.NET 3.0 requirements can be found here:
<http://msdn.microsoft.com/en-us/library/aa480198.aspx>
As far as I know, 3.5 was the last version to support WinXP SP2.