Error: A potentially dangerous Request.Form value was detected from the client

I have a situation where someone has obviously tried to hack into my site.
A server error occurs when someone has placed html-like text into a textbox and submitted.
The error is:
A potentially dangerous Request.Form value was detected from the client (ctl00$MainContent$QuestionTextBox="<a href=" http://chi...").
This happens at a very low level and asp.net sort of traps it and prevents execution of bad data, but it looks like a server crash.
Is there a way to monitor this error, which happens at System.Web.HttpRequest.ValidateString(String value, String collectionKey, RequestValidationSource requestCollection)?
Help most appreciated.