Potentially dangerous Request.Form value was detected

Today i was stucked in one issue after converting my website from .net2.0 to 4.0.

In .net2.0 page is working fine with ValidateRequest="false" but after converting to 4.0 it's throwing error

Sys.WebForms.PageRequestManager.prototype._endPostBack  ScriptResource.axd
 Sys.WebForms.PageRequestManagerServerErrorException:An unknown error occurred while processing the request on the server. The status code returned from the server was: 500

I tried different way to solve the issue but no luck.at the end i tried to remove ValidateRequest="false" attribute from page and then i got actual error which is that "Potentially dangerous Request.Form value was detected"

It was because i am storing some xml values in hidden field.but the strange in .net2.0 it was working fine then what is the issue in 4.0.

I think in .net4.0 Microsoft has change  some logic of validating request of web page.

So currently i have 2 solution to solve the issue.

1. Change the code and remove xml things from hidden variable.
2. I can validate page request as per .net framework 2.0

So Solution no 2 is better for me.

I have added following attribute to web.config

<httpRuntime requestValidationMode="2.0"/>