I use avast free and I use OE6 and have no such issue.
Avast's Mail Shield isn't concerned with spam, only malware, it scans your email and alerts to 'infection.' So is avast alerting ?
I do however use MailWasher Pro for my anti-spam program, so I don't get any spam coming through to OE6 and my inbox as it is filtered out at the email server end.