We had a customer call in today because Outlook 2003 on this Vista computer suddenly wouldn't open. It gave the generic "failed to start" error. After some standard troubleshooting, we discovered the latest iTunes update included an Outlook add-in to allow Outlook to sync with the iPhone. To get Outlook working again, we simply had to disable the add-in, which can be found under "COM Add-ins", not the Add-in Manager.
I'm not sure if this is or will be a common issue, but it's something to look at if Outlook suddenly has a problem and iTunes was recently updated.
You can verify whether an add-in is causing the issue (either the iTunes one, or some other one) by seeing if you can open Outlook in Safe Mode. If Safe Mode works and regular mode doesn't, you can be pretty sure that it's an add-in problem. If it won't open in Safe Mode either... well, then it's probably something else...