MS10-066 is a vulnerability or programming flaw in an application or service.
This error refers to operations that can cause abnormal functioning of the application and can be reproduced artificially by a malicious user in order to take a series of actions including: remote code execution, denial of service, information disclosure or privilege escalation.
A vulnerability is not an immediate threat to the computer. However, it is a potential entry channel for other threats that could have destructive effects. For this reason it is highly advisable to stay informed about vulnerabilities discovered in the programs installed on your computers and apply security patches provided by developers and accessible through their websites.
MS10-066 does not spread automatically using its own means. It needs an attacking user's intervention in order to reach the affected computer. The means of transmission used include, among others, floppy disks, CD-ROMs, email messages with attached files, Internet downloads, FTP, IRC channels, peer-to-peer (P2P) file sharing networks, etc.