It's simple just say to them that they should scan their pc's ... ^.-
a scan might not always do it...
to be absolute sure, scan the computer (Full scan, not some small scan.. include floppy drives)
Change your password of the msn (It can be a "hack" that got your login info and use it to spread links, or it can be stored somewhere there they can check up it and hack your msn so you never can use it again)
And stop download porno, and you should be fine..
Don't click any links you don't know about unless you know how to fight the most nasty viruses there is.
I know tons of website that give you viruses... my avenge amount of viruses try to access my computer use to be 35 per day.
You can seriously get virus from anything from the internet, even ads.. if there's an website that refresh itself (most webs does).. it can change source and suddenly it refreshing (your browser is downloading latest changed of the web) and in that way you downloading a virus.
watch movies onlines... I think I've gotten about 300 viruses from their fucked up ads.. I'm not using that site anymore...
anyway.. if you can't fight viruses... avoid being in webs that nearly "spamm" you with ads.. those are the most dangerous for people that doesn't download stuffs...
Wen you get a mail... check if it's a real mail or a virus shit in it.. would the friend ever mail you? instead of use msn when you're online? and would he really say "Hello mate, can you check this please?" and so on... read between the lines and try understand if it make sense.
Also.. NEVER is hell believe in Anti Virus "ads"... "Your computer are in danger! plx download this software" and similar... 95% of all those ads are fake and contains either virus or fake software to make money out from you that are unexperienced.
if you aren't experienced and not sure wtf you're doing when you going to select antivirus... ask a trustworthy person (take someone you know irl)
anyway, I'm writing too much already and running out of time.
peace out.