Are you sure you want to use XPath for this? It is very simple to use namespaces in LINQ to XML:
XDocument cpo = XDocument.Load(file);
XNamespace x = "http://schemas.microsoft.com/developer/msbuild/2003";
var elements = cpo.Descendants(x + "RegisterForComInterop")
.Where(x => (string) x == "true");
or if you are absolutely sure that each RegisterForComInterop will have a corresponding logical value, you can use the explicit conversion XElementtobool :
XDocument cpo = XDocument.Load(file);
XNamespace x = "http://schemas.microsoft.com/developer/msbuild/2003";
var elements = cpo.Descendants(x + "RegisterForComInterop")
.Where(x => (bool) x);
, XPath, .