Problem with Thread.CurrentPrincipal

I have a problem with custom authentication on ASP.Net.  The website check the login user and password then set the System.Threading.Thread.CurrentPrincipal to an instance of a Principal object implemented IPrincipal Interface.
System.Threading.Thread.CurrentPrincipal = New BusinessPrincipal (ââ?¬Å?userââ?¬Â, ââ?¬Å?passââ?¬Â) 
After this stage th ...