You said every time a user logs on, then you mention when you sign on.
Is this the same account used by you and these other users?
You may find out more by reading those pop up messages yourself.
But I suspect that .Net Framework is required by some Startup software or other and it is this startup software that is failing. Have you, (or has anyone else), installed any software recently that has set itself to load at startup? if so, disable that.
PC: Alienware M11x
OS: Windows 7