Google Search – How to Get Google Currency Converter Back

Google Search, unlike the rest of the Google services, saves the setting to a client-side cookie, rather than a server-side profile. Every time the cookie gets deleted, your search preferences will be reset. This is done to allow the owner of the PC to determine search behavior (such as "safe search" options etc.) regardless of who's using the browser.

You might have a setting in your browser that cleans up cookies upon browser closing, or else, you may have a cleaning or privacy software (i.e. CCleaner) running regularly and removing your cookies.

My recommendation: either stop cleaning the cookies, or save the Google search cookie (it's a text file) and restore it every time you open your browser.

I'm not sure why this is a "problem".

1. Google was "asked" by the MPAA and RIAA to prevent piracy-related terms from showing up in autocomplete and they agreed to do so. Google is not required to show any search suggestions for any particular search term. If you don't like that, you can go use a less-popular search engine, like Bing, that hasn't yet been asked to implement such restrictions.
2. Google has also chosen to hide various terms that are widely considered "NSFW" (Not Safe For Work) from the search suggestions. That way, you can use Google at work without having to worry about what your boss will see on your screen when he comes up behind you.

Note that if you purposely search for these any terms that do not show up in the suggestion list, you will still find them. Google is not censoring your search results.