XML : how to used less han sign in xml document?

I am using C#.net where I required to used xml string,which needs to populate into xmldocument. It is loading fine,but when that string has special following values in one of the node then it is not working sometime I have html tags with style and class. so how to load that string in xml document. so How to deal with in such cases?

here my string which produces an error

  <restdata>   <listingAddress>            fsdfsdf dfdf <Not Specified="">  Argentina dsfsf</listingAddress>   <listingAddress>   xxk dfsdf 899993     </listingAddress>   </restdata>    

in my case error may be because of

also sometime there may be html tags. so how this would be used generalized way so any data my it should work fine?