klez is a worm that can attach to any .exe file and when you open that exe you get klez.. it compresses the exe file then packs junk data at the end of it to make it the same exact size as it was before getting infected plus it closes all antivirus proccess and sometimes deletes the antivirus software itself. it will also check your computer for email adresses and mail itself to them with a header that causes a bug in IE to automatically open the attachment when you read the email even if you didnt want to open attachment.