By mistake, and probably got a virus >_>
Geee thats terrible 
Hope he gets rid of it
Take two of these and call me in the morning. 
Firewall and Anti-virus. 
People really should not use Internet Explorer, Ooutlook express… Those programs are VERY insecure… Instead use for example this mail client:instead of Outlook express: https://seb.mozdev.org/thunderbird/ and one of these browsers instead of IE: https://seb.mozdev.org/firebird or https://www.opera.com/ Not only are this software more secure against viruses and such, but they also have popup filter, tabbed browsing and so on and ontop of that, they are 100% free, with the exception of opera, which has a ad banner on the free version…
Using IE/Ooutlook for normal suring=asking for trouble
Your friend should also consider getting some antivirus and firewall software
I have been using IE for many years and no probs, but i think i might be lucky, because i don’t even receive any spam email (using yahoo email).
BTW what is your friend’s name/nickname?