How to solve A potentially dangerous Request.Form in asp.net?

Actually we are created application using asp.net. after run the asp.net application then user enter * or : or any Html tags in end of the url the enter then throw following  errors,

A potentially dangerous Request.Form value was detected from the client (ctl00$MainContent$txtTextResume="<a href=http://seabr...").

A potentially dangerous Request.Path value was detected from the client (:).

A potentially dangerous Request.Path value was detected from the client (&).

these errors are throws in url

For Ex: www.abc.com\page1.aspx*

          or

         www.abc.com\page1.aspx:

Etc...

 some others sites are handled this.

let me know if any body knows.

Thanks in Advance