Cyberduck 4.2.1 installed on Windows 7 Pro SP1 64-bit machine with Intel i5-3570 CPU. Worked relatively fine yesterday morning--by "relatively," I mean that the app could not connect with an S3-compatible site (objects.dreamhost.com) despite the correct keys. On a predecessor machine, running Windows XP Pro SP3, Cyberduck could connect to the site fine. The bookmark data is identical. In any event, after a few tries, Cyberduck stopped launching. An error popped up, indicating that the program "has recently crashed." The problem details are as follows:

Problem signature:
  Problem Event Name:	CLR20r3
  Problem Signature 01:	cyberduck.exe
  Problem Signature 02:	4.2.1.9350
  Problem Signature 03:	4ee5f3e5
  Problem Signature 04:	System.Drawing
  Problem Signature 05:	4.0.0.0
  Problem Signature 06:	5073c7ec
  Problem Signature 07:	456
  Problem Signature 08:	175
  Problem Signature 09:	StructureMap.StructureMap
  OS Version:	6.1.7601.2.1.0.256.48
  Locale ID:	1033
  Additional Information 1:	0a9e
  Additional Information 2:	0a9e372d3b4ad19135b953a78882e789
  Additional Information 3:	0a9e
  Additional Information 4:	0a9e372d3b4ad19135b953a78882e789

The problem persists even after warm and cold reboots, and even after uninstalling and reinstalling (although I didn't reboot between the uninstall and reinstall...). Nothing was installed between the time the app worked and the time it stopped working.