There is a registry key for this as well: [HKEY_CURRENT_USER\Software\Microsoft\VisualStudio\9.0\Languages\Language Services\C/C++]
Intellisense ON
"IntellisenseOptions"=dword:00000000
Intellisense OFF
"IntellisenseOptions"=dword:00000007
Intellisense ON - NO Background UPDATE
"IntellisenseOptions"=dword:00000005
More flags are available and you can Control Intellisense through Macros as well.
ISENSE_NORMAL = 0 'normal (Intellisense On)
ISENSE_NOBG = &H1 'no bg parsing (Intellisense Updating Off - although NCB file will be opened r/w and repersisted at shutdown)
ISENSE_NOQUERY = &H2 'no queries (don't run any ISense queries)
ISENSE_NCBRO = &H4 'no saving of NCB (must be set before opening NCB, doesn't affect updating or queries, just persisting of NCB)
ISENSE_OFF = &H7