Sounds like your procedures are good and you've nothing to reproach yourself for. It's likely that these e-mails coming in are themselves Spam and can happily be disregarded.
But be careful if you're on ME or XP. I had a bug in the Restore cache that was detected by McAfee but I couldn't figure out how to delete the corrupted files because they were protected. I went the long way round in the end and booted with the recovery disk to a Command prompt and got rid of them that way.
What seems certain is that your address has been "harvested" (like mine) and is now being circulated amonst the low-life who like to waste their lives thinking up new ways to send Spam.
Klez is much smaller than 250 KB normally. But I suppose that it can be packaged in any size the perpetrators want. The minimum size of those I've received would be in the 10-20KB region.