This seems to be a Windows problem and not an office problem. There is a 3rd party app called "Outlook Reminder Tool" that will fix this. Read about it here and decide if you want to use it:
Link Removed - Invalid URL
Or try this suggestion:
The OfficeCommunicator R2 pop up preview feature can be enabled again in Windows 7.
On the task bar arrow select "Customize..." look for Microsoft Office Communicator icon and select "Show Icons and notifications" on the behavior column.