Since your file in _layouts directory, why not to use [GetGenericSetupPath][1] method?
Something like this:

    var path = SPUtility.GetGenericSetupPath(@"TEMPLATE\LAYOUTS\MyPoject\data.xml");

[1]: http://msdn.microsoft.com/en-us/library/microsoft.sharepoint.utilities.sputility.getgenericsetuppath.aspx