Hey.
Improvements/The bad:
I think the navigation menu might look better with labels to guide the user to what they are without having to click to see the content.
The "X" menu button height looks about 1 pixel larger than the minimize one. It might not, but that's how it looks for some reason if you look very closely
You could try seperating the "ANALYZE" and "CLEAN" controls. Maybe put a "|" in a light color between them? It might not look better, I'm not sure unless I see it like that.
I know it's just a concept however I would say "Internet Explorer" comes under the Apps tab.
Last suggestion (for now) is that you could try adding a border on the outside of the application and see what it looks like. If you already have one, my apologies, I just missed it.
Away from the bad:
I do happen to like the FLAT logo and icons, however I think I do prefer the current original ones but I DO like the ones you have done as well. I like both.
I like the "WIN", "APPS", "REG" navigation menu.
I like the title?
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Overall I do and don't like your concept however as a personal preference I do prefer the current CCleaner User Interface. For a first concept I think your going on the right path and with a bit more work and thinking you can start making really nice concepts. Give it some more practise and you'll get even better. And even when someone doesn't like something you have made, don't let that put you down or quit because everyone can get good at something when they put their minds to it and practise enough. Practise will eventually make perfect (well you know what I mean
).