Windows 7 search exact phrase

Hi,

I've got a problem with windows 7 search. I want to search an exact phrase in some files, into search box I insert content:"hello world", but it returns all files containing hello and word, not the exact phrase "hello world".

For example my search returns documents containing

word hello

or

hello friends I live in world

Is it a bug of windows 7 search ?

Thanks

Stefano