Same problem. Have iCloud working on two accounts across 3 macs, 2 iphones, and 2 ipads but cannot get it to work on a Windows 7 machine. I installed, uninstalled, reinstalled the agent for windows a couple of times and i get the same issue as the original poster.
Immediately after the install the agent synchronizes the calendar and contact. That works fine. My icloud contacts and calendar show up in Outlook 2010. But when i make changes to these items they do not get replicated back to icloud. When I go to launch the icloud control panel to try and diagnose the problem i cannot get it to open.
Here is some additional data:
The Outlook plugin is enabled.
There are two processes running: icloud and icloudservices.
The icloud icon shows up in the system tray.
When i right click on the system tray icon and attempt to open the control panel nothing happens.
When I open Windows Control Panel and attempt to launch icloud from there....nothing happens.
If i kill both processes and then click on the iCloud icon in Windows control panel, the processes restart but the iCloud GUI does not open up.