I tried setting up Outlook Express with a message rule to delete messages that have '* ^135AAItEjhyJRI8ci0SOGIlEjxiLRI4UiUSPFItEjhCJRI8Qi0SODIlEjwyLRI4IiUSPCItE' in their body, but it appearently excludes attachments from being considered.
Sorry, I should have explained the Procmail stuff a little better. The text you need to match on is "135AA....SPCItE". The star ("*"), caret ("^"), and dollar sign ("$") are special characters for Procmail and are not actually in the string to be matched. Maybe that'll help, though I can see Outlook skipping attachments, too.
Aparently this string is in the Base-64 encoded Klez binary and unique to its exe.
-Dave