VERY strange autofill occurrence (IE6)

We use IMP here at work, & I just got a very strange call from a user about it. Apparently, my login name, along with the login name of another sysadmin here, was popping up in the "Username" field. This was on two separate machines, which both turned up clean on virus inspection. Both were running IE6, so I figured it best to update them to IE6 SP1 & apply the latest security patches. I also disabled autofill on both setups.

I've come back to my desk, & am scouring the web for other instances of this anamoly with no luck. Then I opened up a browser to check my own setup (I've got autofill disabled), & to my disbelief, my username was in the box. I don't use IMP very often (~5X/month), & I'm up to date on all the patches & am virus-free.

Bottom line: has anyone heard of this happening before? Is this maybe a new virus/worm? This is not happening with Netscape 7.1.