I don't think the problem is the spaces. I think that the problem is that you need to use the proper escape sequence for the backslashes in your path eg.
string ActualPath = "D:\\Files\\a c";
OR
string ActualPath = @"D:\Files\a c";
Try
string ActualPath = @"D:\Files\a c";
DirectoryInfo di = new DirectoryInfo(ActualPath);
if (di.Exists)
{
//do something
}