こんな感じ?
string fullname = Path.GetFullPath(filename);
string[] paths = fullname.Split(':', '\\');
string fullpath = paths[0].ToUpper() + ":\\";
for (int i = 2; i < paths.Length - 1; i++)
fullpath = Directory.GetDirectories(fullpath, paths[i])[0];
fullpath = Directory.GetFiles(fullpath, paths[paths.Length - 1])[0];