Presenting Sample Code

I will be looking for my first 9-5 programming job in a month or so. I am planning to use a small C++ program (syscript) that I''ve been writing as a sample of my work. I have a few questions on how to present this though. First, should I even mention this is my initial contact with a company or should I wait until the interview? If this is in writing, should I provide a written explaination of the code, or should I just let the comments (in the code) do the talking? Is this small (~1500 lines) program sufficient? Is it too large? I''ve used a variety of tools, techniques, and approaches in order to show off my skills while still writing a good program. Should I put comments in the code explaining this reasoning for certain decisions or will most reviewers assume this? Finally, who usually reviews sample code? How deep do they look (i.e. will they look more closely for design flaws or obvious bugs like memory leaks)?