It could be a virus. Have you scanned your pc? If you don't have virus software, you can do it for free here: http://security.symantec.com/sscv6/d...d=ie&venid=sym
I would run a full scan, if anything was detected, I would remove it with your virus software. If you don't have virus protection, then google the files found and find the general consensus way to remove it. Follow all directions VERY VERY carefully, and ask questions when you need to.
Then make sure your windows is updated for security patches, and maybe switch your browser to Firefox. Enable your firewall if you have one, get one if you don't (I can't remember the name of a free one, I think Zone Alarm might be free. My Win XP came with one.)
Oh yeah, also I never ever use Outlook Express, or any email program that downloads the emails to my computer without me even opening them. I use web based email (Yahoo) and never open any kind of attatchments. Also, never download .exe files unless you are damn sure that they are safe. The only time I download an exe is if it's a game patch from the official game site only, or software from a source I can trust (not Joe's list of uber warz).