When you move a file, you need to specify a whole destination file name, not just a folder.
For example:
File.Move(label1.Text, Path.Combine(quarantinePath, Path.GetFileName(label1.Text)))
Always use Path.Combine instead of concatenation - it sorts out trailing backslashes for you.
But ... the reason I used "quarantinePath" as a variable instead of Application.StartUpPath is because you shouldn't try to write to the App program folder - in production that will be below the "Program Files" which is read only without Admin permissions. In an antivirus app, that is especially important, as you would be moving dangerous files into an area that Windows
expects to find executable files in...
Instead of that, use a "sensible" folder, or which there are quite a few. This may help:
Where should I store my data?[
^] - the code is all C#, but it's all very basic stuff.