1. The old syntax outlook/cleanreminders no longer works. First be sure to close Outlook. Then, on the Run command line (click on Start, then Run), you must use the entire path for the Outlook executable (find the path in the Properties for your Outlook shortcut if you have one; verify the correct path including upper or lower case in My Computer or Windows Explorer), surrounded by quote marks since there are spaces in folder names, and including exact upper or lower case (for this purpose the path is case-sensitive), and then add one space and then the switch, such as:
"C:\Program Files\Microsoft Office\OFFICE11\OUTLOOK.EXE" /cleanreminders
Realize you may have to wait a minute or two for the reminder box to pop up, before you conclude that it didn't work.
2. If that doesn't work, try the same syntax with the Outlook command line switch named resetfolders instead of cleanreminders.