Microsoft .NET dis-compatibility

Microsoft .NET dis-compatibility

[new case after solving
http://stackoverflow.com/questions/3620474/microsoft-net-compatibility
I used VS2010, and target for .Net 2.0 (VB.Net Windows Form application) I
ask my XP clients to install .Net 2 or 3.5
This week one of my clients re-installed his XP. I guess that Windows
update installed there .Net 4.
He cannot install .Net 3.5 now, because he gets the following message -
Setup has detected that the Service Pack version of this system is newer
than the update you are applying.
There is no need to install this update.
But there is a need!
Can he uninstall .Net 4 ?
how? Using ControlPanel | addRemovePrograms?
and then install .Net 3.5 and later .Net 4 ?