Outlook auto-complete suggestions cache is stored in <ProfileName>.NK2 file.
By default, it is located in
“C:\Documents and Settings\username\Application Data\Microsoft\Outlook” (Windows XP)
or
“C:\Users\usename\AppData\Roaming\Microsoft\Outlook” (Windows Vista / 7).

To open or export .NK2 files use NK2View utility (More Info).

Outlook 2003
Outlook 2007