not as previous version, outlook 2010 do not use nk2 for autocomplete cache. Now the cache is in the folder of C:\Users\%username%\AppData\Local\Microsoft\Outlook\RoamCache
autocomplete files are looks like Stream_Autocomplete_xxxxxxx.dat.
but simple delete these file will not clear the cache from outlook. the next time you open outlook, these files will be recreated.
win+r run Outlook.exe /CleanAutoCompleteCache
suggest you run it under the folder path of outlook.exe