Welcome to the Security Center Forum,
AVG released a Bart decryptor and you can download it and read instructions how to use it here: http://now.avg.com/barts-shenanigans-are-no-match-for-avg/
If you would like to read more about Bart ransomware and how to remove this infection from your computer please view this guide: http://botcrawl.com/bart-virus-removal-guide/