[DllImport("gdi32", CharSet=CharSet.Auto)]
private static extern int AddFontResource(string lpszFilename);
[DllImport("user32.dll")]
private static extern int SendMessage(int hWnd,uint Msg, int wParam, int lParam);
const int HWND_BROADCAST = 0xFFFF;
const int WM_FONTCHANGE  = 0x1D;

private void btnCheckInstall_Click(object sender,EventArgs e)
{
  string fntName = "aerosol.ttf";
  string fntPath = Path.GetFullPath(Application.StartupPath + 
    "/../../" + fntName);
  bool installiert = FontIsInstalled(fntPath);
  if (!installiert) InstallFont(fntPath);
}

private bool FontIsInstalled(string fntPath)
{
  PrivateFontCollection pfc = new PrivateFontCollection();
  pfc.AddFontFile(fntPath);
  string fntName = ((FontFamily)pfc.Families.GetValue(0)).Name;
  InstalledFontCollection ifc = new InstalledFontCollection();
  foreach (FontFamily ff in ifc.Families)
    if (ff.Name == fntName) return true;
  return false;
}

private void InstallFont(string fntPath)
{
  string fntName = Path.GetFileName(fntPath);
  AddFontResource(fntPath);
  string windir = Environment.GetEnvironmentVariable("windir");
  string fontDir = Path.Combine(windir,"Fonts");
  string fntTarget = Path.Combine(fontDir,Path.GetFileName(fntPath));
  File.Copy(fntPath,fntTarget,true);
  RegistryKey reg = Registry.LocalMachine.OpenSubKey(
    @"SOFTWARE\Microsoft\Windows NT\CurrentVersion\Fonts",true);
  string subkey = fntName.Split('.')[0] + " (TrueType)";
  reg.SetValue(subkey,fntName); reg.Close();
  SendMessage(HWND_BROADCAST,WM_FONTCHANGE,0,0);
}