Okay, let's start off with the UI. First of all, it looks really skiddish and ugly. Personally I'd just use a CLI or maybe a very basic form.
Now that we have that out of the way, let's look at the code.
First of all, your variable names are terrible. Form1? richTextBox1? Come on, give them descriptive names.
Second, separate your core actions from your UI. Instead of having the button2_Click method, have something like VirusTotalScanner.ScanFile(string filepath) which returns a dictionary containing the scanner name and whether it detected the file, then you could just iterate over the results and add them to the listview.
Third, there must be a better way to parse the API result than a regex. I'm sure they use a standard format, so use a parser for it.
I can't even be bothered to review DH_Tools because it looks like just a bunch of barely related functions c/p'd into one big class. I'd instead use a namespace like DH.Tools and group related functions into classes, like reading and writing files into IO. Still, I'd completely scrap the original code because the names make no sense and the actual code is awful. Oh, and your practice of returning "Error" if something fucks up instead of exceptions is generally a bad idea (go Google "why use exceptions" or something).
Get away from malware programming for a bit and go read some books like Clean Code. I think you seriously need to read about the single responsibility principle.
