Anyone know if this is a bug?

[Knipples]

Whenever I try and log into my email, my avast pops up and says there's a virus, and I need to abort the connection.

It says this

File name http://uk.adserver.yahoo.com/a?f=398328001&p=mail&l=REC\{gzip}

Malware name HTML:Iframe-inf

Malware type Virus/Worm

VPS version 091227-0, 27/12/2009

Then I click on abort connection, but then the next time I go to log in or read an email it does it again.

Is it a bug or is it just an advert causing something to be flagged up?

Am running all my scans now BTW