Does anyone happen to know why whenever I open a dialog box like to open or save a file, just before it opens a black window very quickly flashes on the screen and then disappears? It's kind of annoying! This is Windows 7.

In what application you open that dialog box? That black window is probably the Command Prompt window, but this doesn't pop up by itself. You might be infected with something. Try scanning your PC with some good antivirus.

I guess it could be a command prompt, but it happens so quickly that it's hard to see. But I can't be infected with anything because that computer isn't even connected to the Internet and I don't put anything dangerous on it at all.

Open yourself the command prompt and leave it open and then do the dialog box thing. That might cause the new command prompt window to be opened in the one you opened first. Just a guess, don't know if it will work.

Edit:
If that doesn't work, install FRAPS or something to record your screen and you might be able to see the content of that window.

Does it happen with MSPAINT and NOTEPAD as well? If not, then it's probably specific to that program. If so, something probably arched your gap.

Well I don't think it happens with other programs, but I wrote the program and all I told it to do was open a dialog box. And I don't think any text appears in it before it disappears.

Well, it might be useful to see the source, then. Give me the exe and i can check that as well.

I'm really not at liberty to do that, sorry. I'm not trying to be difficult though. I'll try to rebuild it and replace the .exe and see if that fixes it. If it doesn't, then wouldn't that prove that it's somehow just part of the program? I'm using C#, so could this just be a weird thing that .NET does?

I can't see a reason as to why a graphical application would throw up a command prompt.

I don't know enough C# or .NET to be very useful here, and my ideas may be entirely wrong (I'm much more at home with Python, and I primarily code for *nix), but I do remember messing about in VisualStudio several years ago, and somewhere having the option to build for console or graphic environment.

Perhaps the application needs a certain constant defined, or a certain library included to act properly?
Perhaps it's just acting weirdly due to the only thing you have being an open/save window? Might be that the open/save needs a parent window and a default window is created to act as the parent since you didn't explicitly define one.

Well thanks for the suggestion, but a console application is really one in which the whole program runs in a command prompt, and I never said that the whole program was a dialog box - that's just the part that's messing up.

Right, I misunderstood the "all I told it to do was open a dialog box".