Worm_Agent.ARQB arrives as attachment to email messages spammed by another malware or a malicious user. It may be downloaded unknowingly by a user when visiting malicious Web sites.

It drops copies of itself. It creates registry entries to enable its automatic execution at every system startup. It creates registry key(s)/entry(ies) as part of its installation routine.

It uses its own Simple Mail Transfer Protocol (SMTP) engine to send email messages with a copy of itself as attachment. It gathers target email addresses from the Windows Address Book (WAB). It avoids sending email messages to addresses containing certain strings.

It drops copies of itself in folders used in peer-to-peer networks. It also sends copies of itself to target recipients using certain instant messaging applications.

Technical details can be found at this Trend Micro page.