Error:
A potentially dangerous Request.Form value was detected from the client
Scenario:
When you put html tags in your form fields (textbox,textarea) like

This error might occur if you have selected your selection list (dropdownlist) multiple times. You can add multiple=”true” e.g.
in your selection list so as to enable multiple item select.

If you don’t want your selection list to be multiple select enabled then you have to check whether you have selected the selection list multiple times either in your design (aspx file) or in code (aspx.cs file).

Problem

There is a .aspx page with some textbox and a submit button.
User enters text in the textbox and clicks the submit button.

Ok, you installed IIS server in your Windows Operating System.
Then, I suppose you installed .NET Framework.