Out of memory Exception in C#.Net Windows App

Hi,

I am working on a desktop application in C#.Net . Application receives one XML from server , this XML sometimes is  quite big around 5 MB to 10 MB in size. The code is written something like this -

String xml = ((XmlDocument) data).InnerXml;

We get System.OutOfMemoryException when this XML is large . Can any one please suggest any fix how to avoid this exception. I tried using StringBuilder rather than string but this doesn't help. We are using Win7 64 bit OS with 3GB RAM. Any help would be highly appreciated. Thanks in advance.

-Shiv