Well, what is so horribly wrong with the atom logo currently in use?
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It is comparable to the Windows "flying window" logo, makes a great icon, very professional looking, fairly unique, somewhat relates to the name REACTos, and if you stick arms and legs on to it you get a silly office assistant/agent.
If you think about it, Microsoft doesn't have any "mascot" unless you count Rover Retriever (The XP search assistant, originally from MS BOB).
I strongly believe that Windows users are becoming more and more dissatisfied with the unprofessional appearance of Windows XP. Therefore I think ReactOS can pull users away from Windows by remaining professional in appearance and presentation. This includes how it markets itself with logos and mascots.
Besides, Reacty could kick Rovers behind any day.