Try this:
string foo = @"D:\Projects\Some\Kind\Of\Pathproblem\wuhoo.xml";
The problem is that in a string, a \ is an escape character. By using the @ sign you tell the compiler to ignore the escape characters.
You can also get by with escaping the \:
string foo = "D:\\Projects\\Some\\Kind\\Of\\Pathproblem\\wuhoo.xml";