How do install programs with progressbar?

Firstly, check does not change at all inside the loop - you always look at the parameter value. If that is what you want to do, your method can much simpler:

private void checkbox(CheckBox check)
    {
    btnInstall.Enabled = check.Checked;
    }

If that isn't what you need to do, then look at each checkbox inside the loop.

Every time you go round the loop, you check if the current Control is a CheckBox, and if it isn't you disable the button.
So if there are any non-CheckBox controls after the last Checked CheckBox in the form, you will always disable the button regardless of the Checked state.
Move the Disable instruction outside the loop alltogether:

private void checkbox(CheckBox check)
    {
    btnInstall.Enabled = false;
    foreach (Control c in panel.Controls)
        {
        if (c is CheckBox cb)
            {
            if (cb.Checked)
                {
                btnInstall.Enabled = true;
                break;
                }
            }
        }
    }

And yes, you do want to use a BackgroundWorker to do your install - if you don't the UI will freeze up and you'll never see the progress bar change.