Klez infected mail contains a simple HTML page with an embedded empty form.

I don't think this is a response from someone who think you sent them a virus either. The reply goes to Russia, but it came through Brazil. This looks more like an answer to spam.

I got spammed by an outfit called MedicRec last week. They screwed up royally and apparently caused replies (so far all flaming...) to be sent to the whole target list (which then floods the repliers mailbox with bounces from the dud addresses on the list...). Maybe there's another idiot outfit like them out there.