From the Norton write up on the W32.Klez.H@mm
http://www.symantec.com/avcenter/venc/data/w32.klez.h@mm.html

"This worm often uses a technique called "spoofing." When the worm performs its email routine, it can use a randomly chosen address it finds on an infected computer as the "From:" address. Numerous cases have been reported in which users of uninfected computers received complaints that they sent an infected message to
someone else.

For example, Linda Anderson is using a computer infected with W32.Klez.H@mm.
Linda is not using an antivirus program or does not have the current virus definitions.
When W32.Klez.H@mm performs its emailing routine, it finds the email address of Harold Logan. The worm inserts Harold's email address into the "From:" portion of an infected message, which the worm then sends to Janet Bishop. Then, Janet contacts Harold and complains that he sent her an infected message, but when Harold scans his computer, Norton AntiVirus (NAV) does not find anything because his computer is not infected.

If you are using a current version of Norton AntiVirus, have the most recent virus definitions and a full system scan with Norton AntiVirus, which is set to scan all the files, does not find anything, be assured that your computer is not infected with this worm.
There have been several reports that, in some cases, if you receive a message that the virus has sent using its own SMTP engine, the message appears to be a "postmaster bounce message" from your own domain. For example, if your email address is jsmith@anyplace.com, you could receive a message that appears to be from
postmaster@anyplace.com, indicating that you attempted to send an email and the attempt failed. If this is the false message sent by the virus, the attachment includes the virus itself. Of course, such attachments should not be opened."