Let's start with the interjection question. Interjections are punctuated with commas or exclamation points, which is explained at English Grammar Revolution:
Interjections are punctuated with an exclamation mark or a comma. Use an exclamation mark if the emotion is very strong; use a comma if the emotion is not as strong:
Wow! I won the lottery!
Wow, I have a peanut butter and jelly sandwich for lunch.
However, I don't believe you have an interjection in your sentence. As the same website says:
Don't get fooled into thinking that all introductory words followed by an exclamation point or a comma are interjections - they're not!
Hmm... What is an interjection? Do you remember? It is a word that shows emotion. So, if the word in question does not show emotion, it is probably not an interjection.
Maria! Come and see the lion!
Names are not interjections because they do not show emotion. The tone of voice that you say them in may show emotion, but the name itself does not.
Stop! The lion will eat you!
The word stop is not an interjection. It is a verb because it shows action.
So, what part of speech is "No" in your sentence? To answer that, I'll point you toward Wikipedia, which claims that "Yes" and "No" are neither interjections nor adverbs, but parts of speech in their own right:
The words yes and no are not easily classified into any of the eight conventional parts of speech. Although sometimes classified as interjections, they do not qualify as such, and they are not adverbs. They are sometimes classified as a part of speech in their own right, sentence words, word sentences, or pro-sentences, although that category contains more than yes and no and not all linguists include them in their lists of sentence words.
That's just a brief excerpt; I strongly recommend you read the whole article. (The phrase "not all linguists" suggests a lack of consensus, and indicates this can be a thorny – and therefore interesting – problem.)
As for how to punctuate your sentence, I'd recommend a comma. Why do I say that? It looks more natural, it reads more natural, and that seems to be how I've seen it most often printed in books. A simple sample can verify that. If you click on that link, examine the search results, and use that as a guide, you'll find all of the following conventions used:
- No, the new software can still open the old files.
- No. The new software can still open the old files.
- No the new software can still open the old files.
but the first one (i.e., the comma) is by far the most prominent.