Error: Could not load file or assembly 'System.Web.Providers

Hello
I have a Web page with three form fields for:
a) Old password b) New password c) Confirm new password
When I test the fields with aaa (old password), bbb (new password), and bbb (confirm new password) and press 'Submit', I get the following error:
Could not load file or assembly 'System.Web.Providers, Version=2.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35' or one of its dependencies. The located assembly's manifest definition does not match the assembly reference. (Exception from HRESULT: 0x80131040)
The error points to this line in my Web.config file:
Line 43: <add name="DefaultMembershipProvider" type="System.Web.Providers.DefaultMembershipProvider, System.Web.Providers, Version=2.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35" connectionStringName="DefaultConnection" enablePasswordRetrieval="false" enablePasswordReset="true" ...