You could use the following regtweak, which not only disables the search assistant, but also uses advanced search:
CODE
;----- Disable "Search Dog" and use advanced search
[HKEY_CURRENT_USER\Software\Microsoft\Search Assistant]
"Actor"=""
"SocialUI"=dword:00000000
"UsageCount"=dword:00000000
"UseAdvancedSearchAlways"=dword:00000001
If you just use the first two lines, you only disable the search assistant and use the normal search function.