dangerous request.form

Hi all:
 I have a textbox where I look for "<" character. If it is found, show 'invalid character' message
It work fine if he "<" character is between other letters. If it is at the begining I get this error HttpRequestValidationException
Se detectó un posible valor Request.Form peligroso en el cliente (ctl00$cphcontenido$txtcriterio="<s"). It says 'possible dangerous request.form at the client
The only way to solve it I figure is to deactivate it. How to solve it?
Thanks in advanced
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