This is one of the things that you could do in XP (& Vista?) that was removed from Seven. Don't know why it was done, but it was.
If I want a specific few folders to be at the top I name them with Alt+160 as the first "letter". It gives a blank space. Which, I will add, can occasionally cause problems if there's an exe file in there. But for piccies, toons etc. it works OK.