Odd that the registry value would go missing after a reboot. You could try
pasting this into notepad and then save the "C" as 820575.reg. From there,
double click on it to see if it will import into the registry. At this
point, restart Outlook if it is already running and give the customize
button a try.
REGEDIT4
[HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Internet Explorer\Application
Compatibility]
"Outlook.exe"=dword:00000001
The only other thing I can think to check is right click on Outlook Today
and select properties. Go to the folder home page tab and see if the
address of the page starts with a "res:" url.