How can I convert an HTML encoded string to normal string?

If you use .NET 4.5:

using System.Net; // add this at the top of your code file
// the code to decode:
string input = "http%3A%3D%3Dgoogle%2Ecom";
string decoded = WebUtility.UrlDecode(input);

The WebUtility class doesn't exist in older versions, so if you use an older .NET version, add a reference to System.Web and use this code:

using System.Web; // add this at the top of your code file
// the code to decode:
string input = "http%3A%3D%3Dgoogle%2Ecom";
string decoded = HttpUtility.UrlDecode(input);

Note: the above code transforms it into http:==google.com and not http://google.com, because %3D doesn't represent a slash. Use %2F if you want a slash.