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Is this the Ainol Novo 7 you got listed in your profile? When you say "trojan", are you referring to some pre-installed something-or-other you want to delete? As for changing the the ROM, this might be possible. I believe there are some third-party community developed ROMs available. Might be worth having a look at XDA Devs.
you could try reflashing the stock firmware, or using an old backup to restore (if you made one) dont have any experience with that specific device but i found a link for you with the tools youd need. [GUIDE] Flashing/Upgrading the Ainol Novo 7 Venus firmware - Ainol Novo 7 Venus - ZZKKO Forum - THE Forum for Chinese Brands
also, this should probly be moved to the support section for your device so that people who own one can help point you to resources you might need. Good luck!