I would recommend .Net 4.0 that is the latest version supported by Windows XP.
NOTE: All of the .Net framework run times are backward compatible. You also have to make sure that the application you develop is not targeted with a newer version of .Net than the computer with the oldest version of .Net
Here is some useful info:
Windows Vista comes with 2.0 preinstalled.
Windows XP is being fazed out and does not come with .net out of the box, so I would target new computers. I understand if you company is NOT going to upgrade.
Windows 7 comes with 3.5.1 preinstalled. It is compatible up to 4.5.1 (as the time of this writing)
Windows XP comes with none preinstalled but is compatible up to 4.0.